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Heat Resistant Steels

• satyendra
• October 25, 2013
• 3 Comments
• austenite, bainite, bcc, carbides, Carbo-nitrides, Creep strength, farrite, fcc, heat resistant steel, Martensite, nitrides, pearlite,

Heat Resistant Steels

Heat resistant steels are extensively used for high temperature components, and they cover a broad range of applications. The properties of steel and its yield strength considerably decrease as the steel absorbs heat when exposed to high temperatures. Heat resistance means that the steel is resistant to scaling at temperatures higher than 500 deg C.  Heat resistant steels are meant for use at temperatures higher than 500 deg C since they have got good strength at this temperature and are particularly resistant to short and long term exposures to hot gases and combustion products at temperature higher than 500 deg C. These steels are solid solution strengthened alloy steels.

The level of the heat resistance of the heat resistant steels depends on the environmental conditions in which they operate and cannot be characterized by a single testing method. Maximum service temperatures which can be extended to 1,000 deg C depending on the alloy content can be severely reduced by the presence of some compounds such as sulphurous compounds, water vapour, or ash. Resistance to liquid metal and slag is also limited in these steels.

As the heat resistant steels are used over a certain broad temperature ranges, these steels are normally strengthened by hard mechanism of heat treatment, solid solution and precipitation. All the heat resistant steels are composed of several alloying elements for the purpose of achieving the desired properties and are used in applications where resistance to increased temperatures is critical.

High temperature corrosion is a major factor which affect the life of the equipment used for hot service, and the corrosion rate increases as the temperature goes up. In general, increasing the chromium (Cr) content makes materials more corrosion resistant, and corrosion resistance goes up dramatically when the Cr content exceeds 20 %.

The heat resistant steels are normally classified into ferritic / martensitic steels and austenitic steels. The ferritic / martensitic steels have the same body centered cubic (bcc) crystal structure as iron (Fe). They are simply iron containing with relatively small addition of alloying elements, such as the main element Cr added from 2 % to around 13 %. These ferritic / martensitic grades also have a small percentage of manganese (Mn), molybdenum (Mo), silicon (Si), carbon (C) and nitrogen (N2), mostly included for their benefits in the precipitation strengthening and encouraging high temperature behaviour. Ferritic grades are normally used since they are economical because of their low content of alloying elements. They also have some resistance to oxidation at red heat, and which is in direct proportion to the Cr content.

Heat resistant steels are extensively used for high temperature components, and they cover a broad range of applications. Various kinds of heat resistant steels are separately used according to their specific purposes. The heat resistant steels are normally ferritic steels and austenitic steels. Ferritic steels include carbon steels, low alloy steels (0.5 % Mo, 2.25 % Cr-1 % Mo), intermediate alloy steels (5 % to 10 % Cr) and high alloy steels (12 % Cr martensitic steels and 12 to 18 % Cr ferritic steels of the AISI 400 series). Austenitic steels include 18 % Cr–8 % Ni ( nickel) steels and 25 % Cr–20 % Ni steels of the AISI 300 series, 21 % Cr-32 % Ni steels such as Alloy 800H, and Cr-Mn steels of the AISI200 series. Ferritic steels normally do not contain Ni, and, since Cr compositions of 2 %, 9 % and 12 % are particularly high in strength, they are widely used.

Ferritic / martensitic steels used for high temperature service can be divided into two types based on their microstructure and composition. The first type is low alloy steels, which contain 1 % Cr to 3 % Cr and has total alloying elements content less than 5 %. The second type is called 9-12Cr martensitic steels and normally contain alloying elements in the range 10 % to 20 %. As the requirements for the high temperature service becoming more stringent, pearlitic steels are being replaced with the martensitic steels.

In recent years, there is increasing demand for steels which can withstand higher pressure and higher service temperatures. There are development of new grades, which are modification of the existing 9-12Cr grade with additions of vanadium (V), niobium (Nb), and N2. The most advanced martensitic steels today can be exposed to temperatures which are less than 650 deg C.

When sufficient Ni is added to the Fe-Cr steels, the alloy steel becomes austenitic which has a face-centered cubic (fcc) crystal structure. Austenitic steels have high strength and ductility and also have higher creep-rupture strength than the ferritic / martensitic steels. At room temperature, the austenitic steels are more ductile and normally easier to fabricate. Austenitic steels are more expensive because of their high content of alloying elements and are normally used when the temperature is above 650 deg C. Fig 1 shows the bcc and fcc types of the crystal structures.

Fig 1 Type of crystal structures

Heat resistant steels are solid solution strengthened alloy steels for use at temperatures above 500 deg C and upto 750 deg C. As these steels are used over a certain broad temperature ranges, they are normally strengthened by the hardening mechanism of heat treatment, solid solution, and precipitation. In order to achieve the desired properties, all the heat-resistant steels have several alloying elements along with the basic elements. Their complex compositions achieve the outstanding high temperature properties.

In heat resistant steels, the most important alloying elements are Cr for oxidation resistance and Ni for strength and ductility. Other elements are added to improve the high temperature properties. The effect of various alloying elements is described below.

Chromium – Chromium is the one element which is present in all the heat resistant steels. Besides imparting oxidation resistance, chromium adds to high temperature strength and carburization resistance. Cr is the element which makes the micro structure ferritic. Both Cr and Fe have the tendency to form ferrite, and which is counteracted by Ni. High Cr also contributes to the formation of the sigma phase.

Nickel – Ni, when added to the heat resistant steels, increases its ductility, high temperature strength and resistance to both carburization and nitriding. Ni tends to make the atomic structure austenitic. It decreases the solubility of both C (carbon) and N2 in austenite. Iron base alloys become austenitic when a certain amount of Ni is added and hence it is an important element in the austenitic steel. The austenitic heat resistant steels contains at least 8 % Ni.

Carbon – C is the most important strengthening non-metallic element. C is controlled within certain limits in heat resistant steels. When the level of C increases, the steel becomes stronger, but loses its ductility. Majority of the wrought heat resistant steels contain 0.05 % to 0.1 % of C. Cast heat resistant steels have normally 0.35 % to 0.75 % of C. C dissolves in the alloy steel and induces solution strength. It is also present as small, hard particles called carbides which are chemical compounds of C with the metallic elements such as Cr, Mo, Nb, and Ti (titanium) etc.

Nitrogen – N2 is present in heat resistant steels in small amounts and serves to strengthen both martensitic and austenitic steels.

Silicon – Si decreases the solubility of C in the steel. Metallurgically it increases the chemical ‘activity’ of C in the steels, and hence, it is an important variable in the steelmaking process. It is a strengthening element normally above 0.04 %. Si improves oxidation and carburization resistances, as well as resistance to absorb N2 in the heat resistant steels at high temperature.  A silicon oxide (SiO2) layer formed just under the chromium oxide (Cr2O3) scale on the steels helps the steel resist carburization and absorption of N2.

Sulphur – S is normally regarded as an impurity and is normally specified 0.01 % as upper limit in the heat resistant steels. S is detrimental to weldability but it improves machinability. But it has the benefit of improving machinability, so it is kept up around 0.02 % for 304 and 316 grades of steels.

Phosphorus – Phosphorus (P) is normally an undesirable element in heat resistant steels since it has brattling effect when it segregates at the grain boundary. It is also harmful to Ni alloy steel weldability. It is normally specified as upper limit for most of the heat resistant steels. Even the Ni weld fillers are specified to have not more than 0.015 % P.

Other alloying elements – Other alloying elements used in the heat resistant steels are Mn, Mo, V, Ti, Nb, W (tungsten), Al (aluminum), Co (cobalt), Zr (zirconium), Cu (copper), and the rare earth elements like B (boron), Ce (cerium), La (lanthanum) and Y (yttrium). These elements improve the integrative properties of steels at high temperatures. While some of these elements are used for strength others are used largely for oxidation resistance, process workability, and microstructure stability.

A number of studies have been carried out since 1960 on the effects of alloying elements on the creep strength of 9-12Cr steels. Alloying elements for the 9-12Cr steels are easy to understand if they are grouped in terms of their properties and effects into (i) Cr, (ii) Mo, W, and Re, (iii) V, Nb, Ti, and Ta (tantalum), (iv) C and N2, (v) B, (vi) Si and Mn, and (vii) Ni, Cu, and Co.

Cr is the basic alloying element for heat resistant steels, and increased Cr content improves oxidation and corrosion resistance. Although Cr ‘per se’ does not show a marked effect on creep strength, high strength is more likely to be achieved near Cr percentages of 2 % and 9 % through 12% in ferritic steels, and strength declines at compositions between the two coverages. The reason for this remains unknown.

Mo, W and Re (rhenium) are the elements which are useful to solution strengthening. W and Mo have long been used for heat resistant steels. These elements also further improve the creep strength of heat resistant steels when added in larger quantities. However, if their additions exceed a certain limit then delta-ferrite precipitates and reduces the strength, and precipitation of the Laves phase decreases toughness. Also, the effect of W on creep strength is around half that of Mo. The combined addition of W and Mo can be effective for strength improvement. Re is reported to raise creep strength if added in amount of around 0.5 %, and this effect is similar to the actions of W and Mo.

V, Nb, Ti, and Ta all combine with C and / or N2 to produce carbides, nitrides or carbo-nitrides, which finely and coherently precipitate on the ferritic matrix to show a marked effect of precipitation strengthening. Among these, V and Nb are found to show particularly optimal contents, around 0.2 % and 0.05 % respectively. The effect of their combined addition can be high which suggests that the formations of precipitates composed by V and Nb are associated with each other.

Since C and N2 are austenite formers, they are useful in inhibiting delta -ferrite. Also, their contents relate to the precipitation and coarsening of Cr carbides and nitrides. For C particularly, if addition exceeds 0.1 %, the creep strength frequently declines, and it is believed that there is to be an optimal addition according to the types and contents etc., of carbide-forming elements. N2 is believed to be an element essential for raising creep strength in 9 % Cr steels. Addition of N2 is frequently at around 0.05 %, and it is believed that there is to be an optimal content relative to other nitride forming elements such as B.

B improves hardenability and enhances grain boundary strength, and can greatly improve creep strength. Also, studies indicate that it shows the effect of stabilizing carbides by penetrating into M23C6.

With respect to Si and Mn, Si is a ferrite former, whereas Mn is an austenite former. Their actions are viewed as being contradictory to each other, and reduction of the contents of both of these elements can improve creep strength. Also, Si works to decrease toughness by promoting the Laves phase, whereas Mn, though useful for toughness improvement, can impair the high temperature stability of the ferrite structure by decreasing the A1 transformation temperature in the same manner as Ni.

Ni, Cu and Co are all austenite formers, and if added as alloying elements, they inhibit the formation of delta-ferrite by decreasing the Cr equivalent, but they simultaneously decrease the A1 transformation temperature. However, level of this decrease varies among these elements, and the decline seen with additions of Cu and Co is not greater than that found with the addition of Ni. Hence, if Cu and / or Co are added, the effect of the inhibition of delta-ferrite formation can be expected, making high temperature tempering possible.

Allowable stress is a good representation of the high temperature strength characteristics of heat resistant steels, and is frequently determined by creep rupture strength under actual operating conditions. In order to improve the reliability of high temperature components, it is hence necessary to ascertain creep rupture strength upto 100,000 hours (the basis for fixing allowable stress), or to make an accurate estimate of this, and to fully appreciate the relationship between the changes in creep rupture strength and structures over long periods of time. Both high temperature strength and economy is to be considered in the selection of materials. In general, higher cost materials have greater high temperature strength.

Fig 2 shows the chemical compositions of typical heat resistant steels used under stresses in the Fe-Cr-Ni ternary phase diagram. Ferritic / martensitic steels normally do not contain Ni, and, since Cr compositions of 2 %, 9 %, and 12 % are particularly high in strength, they are widely used. Among austenitic steels, materials in commercial use are positioned along the boundary between the full gamma phase and the gamma phase containing alpha phase and / or sigma phase. The full gamma phase steels contain relatively high Ni content and the high cost of which is typically offset by high creep strength. In contrast, the gamma phase steels with alpha and / or sigma phase, though less costly, need some improvement to elevate creep strength.

Fig 2 Compositions of heat resistant steels used in Fe-Cr-Ni ternary phase diagram

Precipitates and the precipitation phenomenon

Heat resistant steels are strengthened by the phenomenon of precipitation. In this regards, complex alloy steels are more advantageous than those simply alloyed, since they have a predominant characteristic of having precipitation reaction. However, the more complex the steel is, the more complicated are the precipitation reactions.

Majority of the precipitates are nitro-carbides, and few of them are inter-metallic compounds. The precipitates are the products of the inter-granular and boundary precipitation as well as variable carbide reactions. The morphology, size, and distribution of these precipitating particles are modified by alloying elements which enhance the properties because of those carbide reactions which are accompanied by microstructural and micro-chemical changes. The types of precipitates present in a specified heat resistant steel depends on its compositions and its heat-treatment. The types of precipitate which are normally formed in the ferritic heat-resistant steels are M23C6, M3C, M2C, M6C, MX, (M stands for metallic solute atom),Laves phase and Z-phase.

There is a simple relationship between carbide structure and the metallic elements. At the service conditions, the most common secondary phases in heat resistant steels are M23X6 carbides, MX carbo-nitrides, Laves phase and Z phase. Hence, a lot of attention has been paid to these precipitates for their notable effects on the creep behaviour.

M23C6 is the main precipitates in the majority of the heat resistant steels. It is believed that the amount of M23C6 and its size and distribution strongly affect the creep strength of heat resistant steels. Normally this carbide is chromium rich, with Fe, Ni, Mn and Mo substitute for Cr partially. The fraction of these substitute atoms can be upto 40 %. The crystal structure of M23C6 is a complex face centered cubic (fcc) which changes slightly for metallic elements variation. As the main carbides in heat resistant steels, M23C6 precipitates mostly precipitate along the grain boundaries during tempering treatment, and some particles can form in the process stage. Even though M23C6 precipitates are very stable precipitation in structure, they coarsen during creep exposure at high temperature. Their average size increases while density decreases when the exposure time increases. The large particles grow by dissolving the fines but volume fraction remains constant.

MX is another typical kind of carbo-nitride precipitate with cubic NaCl-type structure. A large number of fine and dispersive inter-granular MX particles precipitate during tempering and exposure at high temperature when the strong forming elements of V, Ti, Nb and W added in the alloy steels. MX carbo-nitrides normally form on dislocations in the matrix, so they increase creep strength for their dislocation pinning action. Even though MX precipitates are stable and do not coarsens heavily during creep exposure, it is found that they can dissolve and form complex nitride Z-phase under long-term creep exposure.

Z-phase, Cr(V,Nb)N, is another nitride precipitate with similar elements as the MX. Its crystal structure is tetragonal. In high chromium ferritic steels, a Z-phase is found after long-term creep. Additionally, it has been observed that the Z-phase precipitates after creep in both weld metals and the heat affected zone. The discovery of Z-phase precipitation in a 9-12Cr steels raised serious questions about the long term stability. Very little is known about the behaviour of Z-phase in 9-12Cr steels, since it has been observed to precipitate only after long times of exposure or after long term creep testing. Z-phase formation has been recently recognized to decrease the long-term creep strength, since the formation of Z-phase consumes fine MX carbo-nitride particles which are the main strengthening particles in 9-12Cr ferritic steels. The Z-phase precipitates as large particles, which do not contribute to precipitate strengthening, and thus the creep strength of the steel is considerably lowered.

The additions of W and Mo in alloy steels improve their creep behaviour. However, these elements are apt to form intermetallic Laves phase (Fe,Cr)2(Mo,W). Laves phase particles normally precipitate in grain boundaries close to M23C6 carbides in equiaxed shape during creep exposure. The crystal structure of Laves phase is hexagonal with range of composition.

It is found that the size of Laves phase particles increase rapidly in higher exposure temperature. The precipitation of Laves phase leads to a depletion of the two elements Mo and W in the matrix and to a reduction of the solid solution hardening effect in alloy steels. On the other hand, Laves phase precipitation can increase the creep strength by precipitation hardening before it coarsens. It is believed that the Laves phase has a negative effect when its size becomes as one of the largest precipitates in alloy steels.

The type of precipitation present in a specified steel depends on its chemical compositions and heat-treatment. In fact, the heat-resistant steels have shown very complicated metallurgical characteristics. For example, the same steel in different heats or the same steel exposed in different conditions can contain different carbides with dissimilar precipitation, morphology, or distribution.

Types and application of heat-resistant steels

Heat-resistant steels have chemical stability, sufficient strength, and gas corrosion-resistance. These steels can be classified into low alloy steels, martensitic steels, and austenitic steels as per their chemical composition and microstructure.

Low alloy steels– Because of good mechanical properties at high temperatures and sufficient corrosion resistance, low alloy steels are widely used in pressure part applications in boilers. The most recent advancement in low alloy steel is the development of 3Cr-3W(Mo)V steels, which has a higher creep strength than 2.25Cr-1Mo steel and 2.25Cr-1.6W-VNb steel.

In general, Cr-Mo low alloy ferritic steels are tough and ductile at lower operating temperatures and maintain good strength at higher temperatures. Unfortunately, when subjected to prolonged exposure to intermediate service temperatures, these steels can become embrittled with an associated decrease in fracture toughness and a shift in ductile-to-brittle transition temperature (DBTT) to higher temperatures. The embrittlement is mainly caused by changes in the micro-chemistry of grain boundaries, which is referred to as temper embrittlement. Temper embrittlement is non-hardening embrittlement and is caused by grain boundary segregation of impurity elements such as P, Sn (tin), and Sb (antimony) as a result of long term exposure in the temperature range of 350 deg C to 600 deg C. P is considered to be the major embrittling impurity element in steel.

Another type of low alloy steels extensively used for various engineering components are Cr1Mo steels, such as 12Cr1MoV, 14CrMo4-5 (ISO 9328-2, 1991), 13CrMo4-5 (EN 10028-2, 1992), or 12C1.1 (ASTM A182-96) etc. These steels are the heat resistant steels with low additions of alloying elements in chemical composition. These grades are normally used for the pipelines used to transport superheated steam in the temperature range 500 deg C to 560 deg C and under a pressure of 10 MPa to 15 MPa.

The initial microstructure of low alloy steels is ferrite-bainite or ferrite-pearlite. Normally, the Cr-Mo and Cr-W heat resistant steels are used in the normalized and tempered condition. Normalizing consists of heating above A1 equilibrium temperature where ferrite transforms to austenite, and then cooling in air.

In low alloy steels with less than 5 % Cr, bainite (ferrite containing a high dislocation density and carbides), polygonal ferrite, or a combination of these two constituents form, depending on the section size is produced. Their creep strength enhanced by the formation of precipitates, which are stable alloy carbides and intermetallic compounds obtained following normalizing heat treatment later on subjected to very severe tempering (around 700 deg for several hours).

Low alloy steels can contain any of the carbides or carbo-nitrides precipitates such as M3C, M(C, N), M2(C, N), M7C3, M23C6, M6C, Laves phase and intermetallic precipitates. The precipitation sequences at high temperature for some of the low alloy steels are (i) steel grade 2.25Cr1Mo – M3C to M3C + M2C to M3C + M2C + M7C3 to M3C + M2C + M7C3+ M23C6, (ii) steel grade 3Cr1.5Mo – M3C to M3C + M7C3 to M3C + M7C3 + M2C + M23C6 to M7C3 + M2C + M23C6, and (iii) steel grade Cr1MoV – M3C + MC to M3C + MC + M23C6 to M3C + MC + M23C6 + M7C3 to MC + M23C6 + M7C3.

The precipitation sequence in different low alloy steels is noticeably different in the evolution of carbide precipitation even though the thermodynamic driving forces are apparently similar. It is believed that the precipitation sequence difference is related to the changeable driving force for various precipitates in different steels.

During long time service in creep regime to such conditions the microstructure of steel changes, bainite / pearlite decomposes and carbides precipitation at the grain boundaries and carbides coarsening processes proceed. Structure changes cause formation of cavities and development of internal damages. It is because there is a close relation between changes in microstructure and deterioration of mechanical properties. Hence attention is needed to be paid to investigation of the carbides precipitation kinetics of the heat resistant steels during ageing or long-term service at high temperatures. The purpose is to try to determine any microstructural parameters which can be used to estimate service history and can be practicable for assessment of the remaining life of the equipments.

Martensitic steels – Martensitic heat resistant steels include medium Cr steel containing 5 % to 9 % Cr and high Cr steel containing 12 % Cr. The 9-12Cr martensitic steel is used in several applications such as components of boilers and steam turbines. In boilers, these steels are used for tubing in super-heaters and re-heaters, operating with metal temperatures upto 620 deg C. It has been recognized that the 9-12 Cr steels are the key materials to increase the thermal efficiency of steam power plants. In the last three decades, a number of new 9-12%Cr steels with improved creep strength have been developed for long-term service at temperatures close to 650 deg C in high-temperature components of ultra-supercritical power plants.

The steadily improved creep rupture strength of new martensitic 9-12% Cr steels has been used to build new advanced fossil fuel fired steam power plants with higher efficiency. The applications of these new alloy steels achieve not only high efficiencies, but also reduce the emission of CO2 and other environmentally hazardous gases by at least 20 %.

For the long-term application of the new steels, it is necessary to assess the microstructural changes which are likely to take place during service exposure and to evaluate the effect of such changes on the high temperature creep behaviour. This information can help in deciding the design values for the components made from the steels.

In general, martensitic alloy steels have lower coefficients of thermal expansion and higher thermal conductivities than austenitic steels and hence are expected to be more resistant to thermal cycling.

The development of 9-12Cr steels which has taken place around a century ago with the 12 % Cr and 2 % to 5 % Mo steel for steam turbine blades. The high Cr and high C martensitic steels were hard and were subsequently developed commercially for applications such as cutlery knives, razor blades, scalpel blades, and heat-resisting tools and bearings in competition with the austenitic stainless steels.

Beginning with the 12CrMoV steel introduced in power plants in the middle of 1960s, heat resistant steel development over the past decades has led to new steam pipe steels like the modified 9Cr steel P91, introduced in plants in 1988s, to the tungsten-modified 9Cr steels P92 introduced in 2001 and E911 introduced in 2002. Similar steels have been developed and applied for large forgings and castings of steam turbines. The 9-12Cr steels with lower carbon (0.1 % maximum) content and additions of Mo, W, V, Nb, N2 and other elements, possessing higher creep-rupture strengths combined with good oxidation and corrosion resistance at high temperatures, have subsequently been developed. These steels have been used in power plants, petrochemical and chemical plants, gas turbine engineering, aircraft and aerospace industries, and as nuclear fission and fusion reactor components as well.

In order to develop the new alloy steels with advanced characteristics, many efforts have been made to study the metallurgical aspects of the strengthening mechanisms for explaining the improved creep strength. Also, the effects of various additions have been studied. For example, in the course of development of 12CrMoV steels, Ni was added to improve impact properties and to suppress the presence of delta-ferrite in the microstructure. This has been attributed to solid solution hardening and a reduced solubility of C. However, the excessive amounts of Ni, higher than 0.6 %, has caused an accelerated reduction in the creep rupture strength, which can be partly attributed to the reduced stability of M2X phase and the precipitation of M6X particles

Modified 9Cr-1Mo steels have attained their high creep strength by the addition of V, Nb and N2 which form fine precipitates MX, and the creep strength of high Cr martensitic steels have been improved further by replacing part of Mo with W. Specific alloy steels in this class normally include 12 % Cr and 9 % Cr martensitic steels. One famous 12 % Cr steel is X20CrMoV12.1 grade steel. Since the X20CrMoV12.1 grade steel was developed in 1950s, it has been successfully used in power plants for several decades for temperature of around 560 deg C. The creep strength of this grade of steel is based on solid solution strength hardening and precipitation of M23C6 and MX carbides.

Development of heat resistant steels has concentrated on improving the creep properties of 9-12Cr steels for decades. Based on the grade 9, the aim to improve creep resistance has been resulted in the development of new alloy steels such as steel grades 91, 92, E911, 122 and TAF. Fig.3 shows the extrapolated 100,000 hour creep rupture strength for some 9-12Cr martensitic steels as a function of the temperature. The creep rupture property of P92 has been improved obviously.

Fig 3 Creep strength of exposed materials as a function of temperature

An outstanding position is held by the steel with relatively high Cr content of 10.5 % and the B content of 0.027 % to 0.04 %. However, the other attempts to develop high strength alloy steels of this type to promote creep strength have not succeeded because of lack of long-term stability at much higher temperature. It is believed that steam oxidation resistance can limit the maximum operating temperature of these materials, so the alloy steel development is now focused on developing materials with higher Cr contents with a creep strength equivalent to grade 92.

Several studies have shown that the addition of alloying elements appears to be the effective way to improve the creep strength of the 9-12Cr steels. A considerable improvement of creep strength by adding B has been found. Apart from B, Cr is a key element influencing both the oxidation resistance and the creep strength. Experience has shown that 12 % Cr is required to raise the oxidation resistance to the expected level.

Various studies have shown that the improved 9 % Cr alloy steel and 12 % Cr alloy steel can be used for thick section components intended for use in the operating range 565 deg C to 620 deg C. Heat resistant steels of grade 92 and grade 122 are used in the applications where their higher strengths allow the use of thinner sections, thereby reducing the threat of fatigue due to thermal cycling. The reduction in weight brought about by thinner walls also has the effect of reducing stresses at the boiler and turbine connections, as well as on the structural steel work, all of which contributes to increased life of the components and reduced costs.

Results during the development and quality modification of 10 % Cr steels have shown that the fatigue properties of the 10 % Cr material are improved compared to the conventional 11 % to 12 % Cr steels. An added advantage of the 10 % Cr steel compared with conventional low alloy steels is that the lower thermal expansion coefficient allows a greater temperature rise to be tolerated at start-up. Another aim of the development work was to develop 9-12Cr steels for applications upto 650 deg C.

An extensive study of the microstructural development of these alloy steels during thermal exposures has shown a starting microstructure of elongated dislocation cells and sub-grains aligned with M23C6 particles, together with smaller VN (vanadium nitride) and M2X particles inside the sub-grains. During long-duration creep tests some softening of the material occurred in the sample due to the thermal exposure. Some of this softening has been associated with Oswald-ripening of existing particles but precipitation of new particles, in particular Laves phase, has been observed. In parallel, the dislocation density decreased such that few dislocations have been observed inside the sub-grains. All these microstructural changes occur more slowly in steels which contain B.

The microstructure for these 9-12Cr steels is tempered martensite with creep resistance imparted by controlled precipitation of carbides and nitrides. In general, 9-12Cr steels are also used in the normalized-and-tempered condition. Martensite forms when products are normalized by austenitizing above A1 temperature and then air cooled. During this process, the formation of delta ferrite is to be avoided as this can cause embrittlement, resulting in fabrication problems. Subsequent tempering above 700 deg for hours, the strengthening precipitates form. The types of precipitates formed in martensitic heat-resistant steels also depend on the composition, temperature history during fabrication, and time and temperature of service exposure.

The main precipitate in the 9-12Cr steels is the M23C6 carbide consisting of Cr, Fe, Mo, W, and C. This carbide produces the basic creep strength of 9-12Cr steels by precipitating on sub-grain boundaries during tempering. The M23C6 carbides increase creep strength by retarding sub-grain growth, which is a major source of creep strain in 12 % Cr steels. The thermal stability of M23C6 is relatively high. The MX precipitates in 12 % Cr steels consist mainly of V, Nb, and N2, which precipitate within sub-grains where they pin down free dislocations and increase creep strength. Thermal stability of the MX precipitates is very high, leading to high creep strength. It is interesting to note that equilibrium calculations indicate that the strengthening effect of V in the martensite is by precipitation of VN. This means that even though N2 is not a specified alloying element in these steels, the trapped N2 contributes to the high strength.

Austenitic steels – When sufficient Ni (more than 8 %) is added to the Fe-Cr steels, the steel structure transforms to austenitic structure which has a face centered cubic crystal structure. Austenitic steels are stainless steels. They have higher strength, ductility, and creep rupture strength than the ferritic / martensitic steels. Their high toughness makes them insensitive to impact loads and abrupt temperature changes. Austenitic steels are not prone to the grain coarsening at high temperatures. These steels have higher elevated temperature strength as well as creep strength than ferritic steels. At room temperature, the austenitic steels are more ductile, display good formability, and are normally easier to fabricate. These steels are sensitive to sulphurous gases. Machining of these steels is more difficult as compared to ferritic steels.

Austenitic steels are FeCrNi alloy steels with chromium content more than 13 % with an austenitic structure at room temperature. Austenitic steels are more expensive than ferritic steels for their high content of alloying elements. Traditionally, applications for austenitic steels are restricted to the higher temperature areas where severe corrosion conditions exist. Austenitic steels are frequently used as weld overlay on ferritic materials to repair corroded areas or to provide protection in areas where corrosion can be a problem.

Austenitic steels are developed based on 18Cr-8Ni steel which has originated from AISI 302 grade steel. In order to achieve the needed properties, other alloying elements are added. Except the interstitial elements such as C and N2, some substitutional elements such as Mn, Mo, W, Cu, Al, Ti, Nb, V, etc. are added. These alloying elements are classified as ferrite stabilizers or austenite stabilizers by their effects to promote a ferritic structure or an austenitic structure. Their contributions can be evaluated using the notation of Cr and Ni equivalents. Creq (Cr equivalent) is Cr + 2Si + 1.5Mo + 1.75Nb + 1.5Ti + 5.5Al + 0.75W (%) while Nieq (Ni equivalent) is Ni + Co + 0.5Mn + 0.3Cu + 30C + 23N2 (%).

Different types of austenitic steels have been produced by increasing the strength with the addition of alloying elements. Nb, Ti and V can greatly improve the creep strength of austenitic steels by precipitating fine carbides or carbo-nitrides. Also, addition of these elements can stabilize the alloy steel against inter-granular corrosion. Metallurgically, the development of the austenite is to increase the volume fraction of strengthening precipitates by replacing Cr carbides with other more stable carbides, which, at the same time, frees Cr back to the matrix to give improved corrosion resistance. However, various studies have indicated that the presence of secondary phase particles can strengthen austenitic steels, but these secondary phase particles has also been seen to affect the fatigue behaviour.

Austenitic steels such as AISI 316 and AISI 304 grades are used extensively as structural materials in several applications. In fact, austenitic steels are used in many areas, which are subjected to varying temperatures and temperature gradients. Austenitic steels normally have low thermal conductivities and high coefficients of thermal expansion. It is noticed that the high thermal stresses can develop resulting in fatigue cracking.

‘Under-stabilizing’ is one of the techniques for improving the creep strength of 18–8 Cr-Ni steels. This method improves creep strength through improvement of precipitation morphology by fixing C in alloy steels and decreasing carbide forming elements such as Ti and Nb, which obstruct Cr carbide formation, to the point where their contents are insufficient for the C fixation.

Although the Cu addition does not show a major change upto around 2 %, a substantial enhancement in creep strength by means of Cu addition of around 3 % or more can be observed. However, since the strength tends to be saturated, and decline in creep rupture ductility can occur when the Cu addition exceeds 3 %, the addition of Cu at 3 % is to be restricted.

Alloy 800HT is an austenitic Ni-Fe-Cr alloy (21 % Cr-32% Ni-1 % Si-Al-Ti). This alloy is characterized by high creep strength and very good resistance to oxidation. Super austenitic stainless steels 253MA or UNS S30815 and 353MA or UNS S35315 are austenitic Cr-Ni steels alloyed with N2 and rare earth metals. They have high creep strength and very good resistance to isothermal and, above all, cyclic oxidation.

Presently, the development of high strength ferritic steels is being done instead of the austenitic steels because of their cost-effectiveness, good weldability, and fracture toughness, but there is still a place for these austenitic steels which are primarily used in the place where oxidation resistance and fireside creep become more important.

The next generation of heat resistant steels are expected to have even higher yield strength at high temperature, creep strength (typically 100,000 hour rupture strength of around 100 MPa) and high temperature corrosion resistance. However, no present heat resistant alloy can meet these requirements. Hence, presently development work is being carried out to develop steels for use at temperatures upto 700 deg C. One steel which has been successfully developed is the austenitic stainless steel grade UNS S31035. This steel provides very high creep strength and good corrosion resistance at high temperatures. Alloy 800HT is another material developed for use at temperatures upto 700 deg C.

A major concern which remains with the use of austenitic steels is how to join the materials to components manufactured from other material classes. Studies have been carried out for the welding techniques and processes as well as the behaviour of transition joints between the austenitic and martensitic / ferritic steels. It has been found that the location of the failure varied with test parameter. In the high stress at relative low temperature regime failure normally has occurred in the parent ferritic material or weld metal, and failure has been close to the weld interface (heat affected zone) when low stresses were applied over a range of temperatures.

The relatively high costs of the austenitic steels coupled with the disadvantages of high thermal expansion coefficient and poor thermal conductivity, probably continue to limit their applications. There are two possibilities for further development. Firstly, there can be some value in developing a low-cost austenitic steel, possibly based on Mn rather than Ni. Secondly, the modified austenitic materials developed at ORNL (Oak Ridge national Laboratory) can be introduced commercially. The high strength capability of these alloys can give advantages in terms of thinner sections and improved heat-transfer characteristics.

As to other heat resistant alloys which can be used in the future, ‘oxide dispersed strengthened’ (ODS) alloys are promised, which are characterized by good creep strength at high temperatures (i.e. higher than 1,000 deg C). Hence these alloys are seen as being useful for higher temperature applications where the strength of the super-alloys was inadequate. These alloys are normally produced by mechanical alloying. Since the processing route is complex and difficult to control in large-scale processing and as a result these alloys are characterized by poor reproducibility of microstructure and inconsistency of mechanical properties. ODS alloys can be classed in iron base and Ni base for the difference of main composition. Studies have indicated their good high temperature oxidation resistant behaviour is correlated with the dispersive oxide, such as Y2O3. It is believed that ODS alloys are most beneficial in areas such as heat exchanger tubing where high temperatures are normally found but the loading is low in comparison to other areas. Efforts to improve strength and to develop better fabrication techniques are continuing, and other major effort is towards the establishment of reliable joining techniques.

Microstructure evolution in in-service materials

Microstructures of 9-12Cr steels presently being developed or already commercially available consist of a single phase of tempered martensite, with some exceptions. High density dislocations exist in this structure, and the dislocation density is principally influenced by the tempering temperature. It becomes high when the tempering temperature is low.

It is believed that creep and material structure evolution are tightly related. Components subjected to creep stress have a limited lifetime. In fact, a large number of studies have been performed in order to relate microstructural investigation and service exposure or residual life.

The creep properties of heat-resistant steels are controlled by chemical composition and microstructure of these steels. If the chemical composition is given, the microstructure of these steels depends on the heat treatment, temperature, and time of creep exposure. The most important strengthening mechanisms in these steels, operating during high temperature creep exposure, are precipitation strengthening and solid solution strengthening. Precipitation strengthening in ferritic steels is predominantly affected by the dispersive MX particles. It has been shown that both the proof stress at room temperature and creep rupture strength increase while density of second phases increase. At the same time, the creep rate decreases.

If the tempering temperature is low, the creep rupture strength in the short term region is typically high, whereas it rapidly decreases in the long term region, and the strength time to rupture curve is crossed over by that of high temperature tempered steels. This is caused probably because in low temperature tempered steels, recrystallization from martensite to equi-axed ferrite occurs during creep, thereby rapidly dropping the strength, whereas high temperature tempered steels have microstructures where the dislocation density of martensite is too low in terms of tempered conditions to derive recovery and recrystallization. From these microstructural observations, weakening due to the change in microstructure is less likely to occur during creep in the case of unstable structures. The same is known to be true for 12 % Cr steels, and tempering has hence been conducted in recent years at a temperature of around 700 deg C, although the tempering temperature for these steels was formerly around 650 deg C.

It is clear that the mechanical properties of heat-resistant steels deteriorate when it is exposed at high temperature for long-term. Correspondingly, their microstructure also degrades obviously. As to the characteristic of microstructure evolution, the key phenomena of degradation are (i) precipitates coarsening and phase transformation, (ii) the original microstructure decomposition, and (iii) micro-voids forming at grain boundaries. These microstructural evolutions directly connect the deterioration of creep strength and other properties, so they are frequently considered as a demonstration of overheating exposure, and have been normally accepted as a qualitative thermal degradation index. Studies carried out in past decades have shown that different class of materials have different characteristic in microstructure evolution.

Microstructure evolution in ferritic steels – The low alloy steels or pearlite / bainite steels show the tendency for the spheroidization of pearlite / bainite after long-term exposure at high temperature. The typical lamellar structure in CrMoV pearlite change to particle structure. Service exposure has a considerable effect on the strength and ductility. Classification of microstructural deterioration in steels has also been established and standardized. Five levels of degradation are assigned based on the development of carbide spheroidization. The classification specifies Level 1 as having no spheroidization, Level 2 as having slight spheroidization, Level 3 as having medium spheroidization, Level 4 as having complete spheroidization, and Level 5 as having serious spheroidization. From Level 1 to Level 5, the lamellar structure in pearlite changes to a particle structure. The pearlite structure disappears at Level 4 and Level 5, which results in deterioration of mechanical properties. Hence, the classification of microstructural deterioration in low alloy steels is established by the spheroidization grade.

Several studies have been conducted on the evolution of carbides present in steels because of the creep exposure. Separation and coarsening of carbides are considered as an index of material degradation due to the creep exposure. Microstructure evolution in 9-12Cr martensitic steels shows much complicated characteristics during long-term exposure at high temperature. Various studies have shown that much more attention is paid to precipitates coarsening, martensitic substructure transformation and micro-voids formation at the stage when steel is degraded. These studies present estimation of average particle size and remaining life fraction for ferritic steels. In these studies more attention has been given to M23C6, VC, Laves phase and their compositions. However, different results of particle size and composition have been achieved from different studies in terms of absolute size value. Statistical measures have a good agreement in terms of correlation of average particle size on temperature exposure or on time maintaining at a certain temperature. This significant difference in terms of absolute values (size, content, inter-particle space etc.) is not only strongly influenced by statistical approach in estimating average quality, accuracy in measurement at high magnification and sampling technique adopted, the exposure history of the examined component and its metallurgical factors and forming process are essential actions.

Unlike low alloy steels, martensite infrequently is less sensitive to form inter-granular cavities and the impact factors for cavities formation is much complicated, but some evidence of creep damage can be seen in sub-structure. The shapes of the laths are changed, in particular, the lath boundaries look like bamboo knots, called cell structure, which is a typical microstructure morphology caused by creep. Many low dislocation density regions appeared in the lath structure, and some typical substructures can be seen. The substructure seems to develop as sub-grains boundaries are formed by dislocation movement during the creep process. A substantial reduction of dislocation density is observed, and few dislocation-free regions can be seen. Extensive carbide precipitates can be seen at prior austenite and martensite lath boundaries, with the finer precipitates in martensitic laths. Large coarsening carbides in irregular spheroid formed along the boundaries. Compared with virgin material, the carbide morphology coarsened distinctly.

The observations during a study has indicated that the matrix of the tempered martensite has undergone a deterioration during long-term creep. The dislocations climbed or glided and terminated at boundaries. As the number of dislocations at the boundaries is increased, networks are formed and sub-structures are developed. The carbides morphology in boundaries coarsened distinctly, and most of the strengthening phase have dissolved or coarsened. Except the coarsening characteristic, martensitic steels have shown normal evolution in microstructure which is martensite decomposition and substructure change during long-term exposure at high temperature.

It is normally accepted that martensitic structure is under a degeneration with creep exposure prolonging. The dislocation density decreases and the precipitates coarsening and the density of fine particles within the matrix decreases during long-term exposure. The martensitic lath boundaries is indistinct or even disappear since lath has undergone a degeneration. Coarsening of laths is because of two ways namely (i) the recovery of dislocations within lath boundaries, and (ii) the recombination of two sub-grain boundaries which mainly takes place near the triple point of lath boundaries by moving and causes the disappearance of lath boundaries. Also, during moving of lath boundaries to cause progressive local-coalescence, dissolution and re-precipitation of M23C6 carbides distributing along lath boundaries take place repeatedly. However, it is not confirmed that the MX precipitates evolve during creep or thermal exposure.

The solid solution strengthening effect by alloying elements is weakened but was found to be negligible. The precipitation of Laves phases do not strongly affect resistance of this steel to creep deformation, however, that damage cavities are frequently found next to Laves phases, so that Laves phases can affect the resistance of the steel to creep fracture. It is normally accepted that there is the increase of Cr content in M23C6 precipitates in case of ferritic steels, and the increase of Mo content in precipitate in eutectoid carbide as M3C or as M6C. The composition variation in precipitates and inter-particle distance decrease with carbides coarsening are the result of creep exposure and microstructural degradation.

However, there is a notable difficulty in correlation of microstructural evolution (such as second phases) to residual life assessment. It is not only necessary to know the actual state of original material which for the same type of material can vary considerably from heat to heat or product to product, but the exposure condition and history since these are also the factors which affect the evolution course. Moreover, there is no quantitative relationship has been established to correlate the parameters of microstructural evolution with life depletion.

Micro-voids formation in grain boundaries– The concept of micro-void or micro-cavity formation at the grain boundaries has been studied and developed in the1970s. It is normally recognized and applied with the Neubauer classification and derived methods for decades. The principle is based on the fact that creep evolution of heat resistant steels is related to the appearance of cavities some time before rupture. These cavities gradually form micro-cracks by inter-linkage and at the end come to initiate the rupture. The emergence of micro-voids means that the steel in service has damaged severely.

Neubauer schematic assessment of the microstructure has a number of classes namely (i) class 0 which is as received, without thermal service load, (ii) class 1 which is creep exposed, without cavities, (iii) class 2a which is advanced creep exposure, isolated cavities, (iv) class 2b which is more advanced creep exposure, numerous cavities without preferred orientation, (v) class 3a which is creep damage, numerous orientated cavities, (vi) class 3b which is advanced creep damage, chains of cavities and / or grain boundary separations, (vii) class 4 which is advanced creep damage, micro-cracks, and (viii) class 5 which is large creep damage, macro-cracks.

Another important aspect of correlation of cavities presence to creep progress can be found in the Neubauer documents, where it is stated that ‘a noticeable cavity formation takes place at grain boundary at the end of secondary creep’.  Since examples of damage grade allocation seem to be not completely corresponding with the creep curve, it can be considered that class 4 and class 5 can be taken as representative of different stages of tertiary creep, while class 3 is taken as the transition point among secondary and tertiary and class 2 considered as representative of secondary creep.

The size and density of the cavities normally increase as creep progresses. The size and density of the cavities are also dependent on material type, however it is normally accepted that the formation of the micro-voids and their evolution figure is a prominent index to estimate damage degree. Studies on the micro-void formation indicate that cavity nucleation is associated with grain boundary and second phase particles in it, and the presence of surface active elements such as P and Sn in grain boundary which makes cavity nucleation easier. The density of cavities (number of cavities per unit grain boundary area) increases with creep exposed time and temperature, and the applied stress enhances the cavity density too. Cavities growth is controlled by two mechanisms: diffusion growth and constraint growth, the former is dominating at high stress level, and the latter one is dominating at much lower stresses.

The size of micro-void is in the range of micrometers and it cannot be detected by conventional NDT (non-destructive testing) techniques, so metallographic investigation is needed to observe its morphology and distribution.

Some important points related to heat resistant steels

Selection of heat resistant steels for a particular application is based on the level of the heat resistance required and the needed mechanical properties from the steel. The use of a higher alloying elements and hence more heat resistant can be disadvantageous because of embrittlement besides having a higher cost. Heat resistant steel is not to be exposed to flame and a direct contact with C is to be avoided to prevent the lowering of heat resistance due to carburization.

Heat resistant steels are used in industrial furnaces, steam boilers, steam pipes, steam turbines, recuperators,  metallurgical, chemical and petroleum industries, gas and fuel lines, fire boxes, heaters, resistors, heat exchangers and waste incineration plants etc.