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Steel Castings

• satyendra
• July 27, 2020
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• carbon steel, cast steel, corrosion resistance, High temperature property, low alloy steel, Low temperature property, machinability, Soundness, steel casting, toughness, Wrought steel,

Steel Castings

Steel castings are produced by pouring liquid steel of the desired composition into a mould of the desired configuration and allowing the steel to solidify. The mould material can be silica, zircon, or chromite sand, olivine sand, graphite, metal, or ceramic. The choice of mould material depends on the size, intricacy, dimensional accuracy of the casting, and the cost. While the producible size, surface finish, and dimensional accuracy of castings vary widely with the type of the mould, the properties of the cast steel are not affected significantly.

Steel castings can be made from any of the many types of carbon (C) and alloy steels produced in wrought form. Those castings produced in any of the various types of moulds and wrought steel of equivalent chemical composition respond similarly to heat treatment, have the same weldability, and have similar physical and mechanical properties. However, cast steels do not show the effects of directionality on mechanical properties which are typical of wrought steels. This non-directional characteristic of cast steel mechanical properties can be advantageous when service conditions involve multi-directional loading. Another difference between steel castings and wrought steel is the de- oxidation needed during steelmaking. Cast steels are made only from fully killed (deoxidized) steel in a foundry, while wrought products can be made from rimmed, semi-killed, or killed steel ingots or killed continuous cast steel in a rolling mill. The method of producing the killed steel used for a casting can also differ from that used for a wrought steel product because of the differences in the tapping temperatures required in the steel casting and ingot or continuous cast steel production.

However, the salient features of producing killed steel in a cast steel foundry are the same as those features important to the production of fully killed steel ingots or continuous cast steel products. For de-oxidation of C and low alloy steels (i.e. for control of their oxygen content), aluminum (Al), titanium (Ti), and zirconium (Zr) are used. Of these, Al is used more frequently because of its effectiveness and low cost. Unless otherwise specified, the normal sulphur (S) limit for C and low alloy steels is 0.06 %, and the normal phosphorus (P) limit is 0.05 %.

Classifications and specifications

The steel castings are normally divided into four general groups according to the composition. These are (i) low C steel castings, (ii) medium C steel castings, (iii) high carbon steel castings, and (Iv) low alloy steel castings. Other types of steel castings are heat-resistant castings, stainless (corrosion-resistant) steel castings, and austenitic manganese (Mn) steels castings.

Carbon steels contain only C as the principal alloying element. Other elements are present in small quantities, including those added for deoxidation.  Mn and Si (silicon) in cast C steels typically range from 0.5 % to around 1 % of Mn and 0.25 % to around 0.8 % of Si. Like wrought steels, C cast steels can be classified according to their C content into three broad groups namely (i) low carbon steels (0.2 % C or less), (ii) medium C steels (0.2 % to 0.5 % C) and high C steels (0.5 % C or more). C content is a principal factor affecting the mechanical properties of the cast steels. The other important factor is the method of heat treatment.

The basic trends of the mechanical properties of C cast steels as a function of the C content for four different heat treatments are given in Fig 1. For a given heat treatment, a higher C content generally results in higher hardness and strength levels with lower ductility and toughness values.

Fig 1 Properties of cast C steels as a function of the C content and heat treatment

Low alloy steels contain, in addition to C, alloying elements upto a total alloy content of 8 %. Cast steels containing more than the amounts given in Tab 1 of a single alloying element are considered low alloy cast steels. Al, Ti, and Zr are used for deoxidation of low alloy steels. Of these elements, Al is used most frequently because of its effectiveness and low cost.

Several types of cast low alloy steel grades exist to meet the specific requirements of the end-use, such as structural strength and resistance to wear, heat, and corrosion. The designations used to identify the various types of steel by their principal alloy content are given in various standards. Cast steels, however, do not precisely follow the compositional ranges specified by the designations for wrought steels. In most cases, the cast steel grades contain 0.3 % to 0.65 % Si and 0.5 % to 1 % Mn, unless otherwise specified. The principal low alloy cast steels are Mn steels, Mn-Mo steels, Mn-Mo-B (boron) steel, Ni steel, Ni-Cr-Mo steel, Mn-Ni-Cr-Mo steel, and Cr-Mo steel. There are additional alloy types which are infrequently specified as cast steels, for example, Ni-Cr, Mo, Cr, Cr-V, Ni-Mo, and Si steels.

Steel castings are normally being produced to meet specified mechanical properties, with some restrictions on chemical composition. Unless otherwise noted, all the grades of cast low alloy steels are restricted to P content of 0.4 % maximum and S content of 0.045 % maximum.

In the low strength ranges, some specifications limit C and Mn content, normally to ensure satisfactory weldability. C and Mn are normally specified to ensure that the minimum desired hardness and strength are obtained after heat treatment. For special applications, other elements can be specified either as maximum or minimum, depending on the characteristics desired.

The requirements for the low-alloy classes of steel castings are given in some of the specifications. In addition, some specifications may address common requirements of all steel castings for a particular type of application. As an example, one specification specifies the general requirements of steel castings for pressure-containing parts. If only mechanical properties are specified, the chemical composition of castings for general engineering applications is normally left to the discretion of the casting producer. For specific applications, however, certain chemical composition limits have been established to ensure the development of specified mechanical properties after proper heat treatment, as well as to facilitate welding, uniform response to heat treatment, or other requirements. Hardness is specified for most of the grades to ensure machinability, ease of inspection for high production rate items, or certain characteristics pertaining to wear.

The cast steel specification includes three grades, HA, HB, and HC, with specified hardenability requirements. The hardenability requirements for these steels, both minimum and maximum, are given in Fig 2. Hardenability is determined by the end-quench hardenability test. Some specifications of cast steel require minimum hardness at one or two locations on the end-quench sample. In general, hardenability is specified to ensure a predetermined degree of transformation from austenite to martensite during quenching, in the thickness required. This is important for critical parts requiring toughness and optimum resistance to fatigue.

Fig 2 End-quench hardenability limits for the hardenability grades of cast steel 

Among the two most commonly selected grades of steel castings are, first, a medium C steel and second, a higher strength steel, often alloyed and fully heat treated. Particularly when the user heat treats a part after other processing, a casting is to be specified to compositional limits closely equivalent to the wrought steel compositions, with somewhat higher Si content permitted. As in other steel castings, it is best not to specify a range of Si, but to permit the foundry to utilize the Si and Mn combination needed to achieve required soundness in the shape being cast. The Si content is frequently higher in cast steels than for the same nominal composition in wrought steel. Si above 0.8 % is considered an alloy addition since it contributes significantly to resistance to tempering.

Some major users of steel castings can prefer their own or industry association specifications. Users of steel castings for extremely critical applications, such as aircraft, can use their own, industry association, or special-purpose military specifications. Foundries frequently make non standard grades for special applications or have their own specification system to meet the needs of the user. Savings can be realized by using a grade which is standard with a foundry, especially for small quantities.

Low C cast steels

Low C cast steels are those steels which are having a C content of less than 0.2 %. Most of the quantity produced in the low C cast steels contain between 0.16 % C and 0.19 % C, with 0.5 % to 0.8 % Mn, 0.05 % maximum P, 0.06 % maximum S, and 0.35 % to 0.7 % Si. In order to obtain high magnetic properties in electrical equipment, the Mn content is normally held between 0.1 % and 0.2 %. The properties of these electric steels can be slightly below those of typical low C cast steels because of their Mn content.

Fig 1 includes the mechanical properties of C cast steels with low C contents within the range of around 0.1 % to 0.2 %. There is very little difference between the properties of the low C steels resulting from the use of normalizing heat treatments, and the properties of those which are fully annealed. In cast steels, as in the case of rolled steels of the same composition, increasing of the C content increases strength and decreases ductility. Although the mechanical properties of low C cast steels are nearly the same in the as-cast condition as they are after annealing, low C steel castings are often annealed or normalized to relieve stresses and refine the microstructure.

Low C steel castings are made in two important classes. One can be termed railway castings, and the other miscellaneous jobbing castings. The railway castings consist mainly of comparatively symmetrical and well-designed castings for which adverse stress conditions have been carefully studied and avoided. Miscellaneous jobbing castings present a wide variation in design and frequently involve the joining of light and heavy sections. Varying sections make it more difficult to avoid high residual stress in the as-cast shape. Because residual stresses of large magnitude cannot be tolerated in many service applications, stress relieving becomes necessary. Hence, the annealing of those castings is decidedly beneficial even though it can cause little improvement of mechanical properties. Castings for electrical or magnetic equipment are usually fully annealed since it improves the electrical and magnetic properties.

An increase in mechanical properties can be obtained by quenching and tempering, provided the design of the casting is such that it can be liquid quenched without cracking. Impact resistance is improved by quenching and tempering, especially if a high tempering temperature is used.

The uses of low C cast steels include important castings for the railways. Some castings for the automotive industry are produced from this category of steel, examples are annealing boxes, annealing bottoms, and hot metal ladles. Low C steel castings are also produced for case carburizing, by which process, the castings are given a hard, wear-resistant exterior while retaining a tough, ductile core. The magnetic properties of this category of steel make it useful in the production of electrical equipment. Free-machining cast steels containing 0.08 % to 0.3 % S are also produced in low C steel grades.

Medium C cast steels

The medium C grades of cast steel contain 0.2 % to 0.5 % C and represent the bulk of steel casting production. In addition to C, they contain 0.5 % to 1.5 % Mn, 0.05 % maximum P, 0.06 % maximum S, and 0.35 % to 0.8 % Si. The mechanical properties at room temperature of cast steels containing from 0.20 % to 0.5 % C are included in Fig 1. Steels in this C range are always heat treated which relieves casting strains, refines the as-cast structure, and improves the ductility of the steel.

Unlike low C steel castings, when medium C steel castings are fully annealed, it is possible to increase the yield strength, the reduction of area, and the elongation over the entire range, compared to as-cast properties (Fig 3). This increase is pronounced for steel with a C content between 0.25 % and 0.5 %. The hardness and tensile strength can be expected to fall off slightly following full annealing. A very large proportion of steel castings of this grade are given a normalizing treatment, followed by a tempering treatment. The improvement in mechanical properties of medium C cast steel which can be expected after normalizing or normalizing and tempering is shown in Fig 1.

Fig 3 Effect of annealing on the mechanical properties of medium C steel castings

If the design of a casting is suitable for liquid quenching, further improvements are possible in the mechanical properties. In fact, to develop mechanical properties to the fullest degree, steel castings are to be heat treated by liquid quenching and tempering. Commercial procedure calls for tempering to obtain the desired strength level. Tempering temperatures of 650 deg C to 705 deg C are normally used to get higher ductility and impact properties.

High C cast steels

Cast steels containing more than 0.5 % C are classified as high C cast steels. This grade also contains 0.5 % to 1.5 % Mn, 0.05 % maximum P, 0.05 % maximum S, and 0.35 % to 0.7 % Si. The mechanical properties of high C steels at room temperature are shown in Fig 1. High C cast steels are frequently fully annealed. Sometimes, a normalizing and tempering treatment is given, and for certain applications an oil quenching and tempering treatment can be applied.

The microstructure of high C cast steel is controlled by the heat treatment. C also has a marked influence, as an example, giving 100 % pearlitic structure at eutectoid composition (around 0.83 % C). Higher proportions of C than eutectoid composition increases the pro-eutectoid cementite, which is detrimental to the casting if it forms a network at the grain boundaries because of improper heat treatment (for example, slow cooling from above the Acm temperature). Faster cooling prevents the formation of this network and, hence, improves the properties.

Low alloy cast steels

Low alloy cast steels contain a total alloy content of less than 8 %. These steels have been developed and used extensively for meeting special requirements which cannot be normally met by ordinary plain C steels with low hardenability. The addition of alloys to plain C steel castings can be made for any of several reasons, such as to provide higher hardenability, increased wear resistance, higher impact resistance at increased strength, good machinability even at higher hardness, higher strength at high and low temperatures, and better resistance to corrosion and oxidation than the plain C steel castings. These steel castings are produced to meet tensile strength requirements of 485 MPa to 1,380 MPa together with some of the above special requirements.

Alloy cast steels are used in machine tools, high speed transportation units, steam turbines, valves and fittings, pulp and paper machinery, refinery equipment, rayon machinery, as well as railway, automotive, excavating, and chemical processing equipment, and various types of marine equipment. These steels are also used in the aeronautics application.

Low alloy cast steels can be divided into two classes according to their use. These classes are (i) those used for structural parts of increased strength, hardenability, and toughness, and (ii) those resistant to wear, abrasion, or corrosive attack under low temperature or high temperature service conditions. There can be no sharp distinction between the two classes since many steels serve in both the areas.

The present trend toward decreasing weight through the use of high strength materials in lighter sections has had a marked effect on the development of low alloy cast steels. Low alloys grades of several families, are capable of producing mechanical properties with a yield strength 50 % higher and a tensile strength 40 % higher than C steels, with a ductility and impact resistance at least equal to unalloyed steels. Some 75 to 100 combinations of the available alloying materials have been regularly or occasionally used. It is doubtful that this many variations in composition are necessary or economical.

Types of low alloy steels

The compositions of low alloy cast steels are primarily characterized by C contents under 0.45 % and by small amounts of alloying elements, which are added to produce certain specific properties. Low alloy steels are applied when strength requirements are higher than those obtainable with C steels. Low alloy steels also have better toughness and hardenability than the C steels.

C-Mn cast steels – Mn is the cheapest of the alloying elements and has an important effect in increasing the hardenability of steel. For this reason, many of the low alloy cast steels now contain between 1 % Mn to 2 % Mn. In the normalized steels in which grain refinement is also needed, V, Ti, or Al is frequently added. C-Mn steels containing 1 % to 1.75 % Mn and 0.2 % to 0.5 % C have received considerable attention in the past because of the excellent properties which can be developed with a single, relatively inexpensive alloying element and by a single normalizing or normalizing and tempering heat treatment. C-Mn steels are also referred to as medium Mn steels and have Mn content in the range of 1.6 % to 1.9 % Mn.

Mn-Mo cast steels – These steels are very similar to the medium Mn steels with the added characteristics of high yield strength at higher temperatures, higher ratio of yield strength to tensile strength at room temperature, greater freedom from temper embrittlement, and greater hardenability. Hence, these steels have replaced medium Mn steel for certain applications. There are two general grades of Mn-Mo steels. The first grade contains 1 % Mn to 1.35 % Mn and 0.1 % Mo to 0.3 % Mo. The second grade contains 1.35 % Mn to 1.75 % Mn and 0.25 % Mo to 0.55 % Mo. For both of these alloy types, the selected C content is often between 0.2 % and 0.35 %, depending on the heat treatment employed and the strength characteristics desired.

Mn-Ni-Cr-Mo cast steels – These low alloy steels are primarily produced for their high hardenability. Sections exceeding 125 mm in thickness can be quenched and tempered to get a fully tempered martensitic structure. The composition range used for these steels is Mn content ranging from 1.3 % to 1.6 %, Ni content ranging from 0.4 % to 0.7 %, Cr content ranging from 0.55 % to 0.75 %, and Mo content ranging from 0.3 % to 0.4 %. Ni or Mo with Mn refines the grain structure to a lesser extent than does V, Ti, or Al, but each is important for increasing the ability of the steel to air harden. Cr and V impart considerable hardenability. V containing steels are sometimes precipitation hardening and, hence, can have higher tensile and yield strengths.

Ni cast steels – Among the oldest alloy cast steels are those containing Ni. Ni and Ni-V steels are used for parts exposed to sub-zero conditions (such as return headers, valves, and pump castings in oil-refinery dewaxing processes) because of good notch toughness at lower temperatures. These steels are characterized by high tensile strength and elastic limit, good ductility, and excellent resistance to impact. The Ni cast steels typically contain 2 % to 4 % Ni, depending on the grade needed.

Ni-V and Ni-Mn cast steels – These cast steels are used for structural purposes requiring wear resistance and high strength. The Mn- Mo cast steels are also used in these applications.

Ni-Cr-Mo cast steels – The addition of Mo to Ni-Cr steel considerably improves hardenability and makes the steel relatively immune to temper embrittlement. Ni-Cr-Mo cast steel is particularly well suited to the production of large castings because of its deep-hardening properties. In addition, the ability of these steels to retain strength at high temperatures extends their usefulness in many industrial applications.

Cr-Mo cast steels – Cr contents of around 1 % or more provide a nominal improvement in elevated-temperature properties. Cast steels containing Cr, Mo, V, and W have given good service in valves, fittings, turbines, and oil refinery parts, all of which are subjected to steam temperatures upto 650 deg C. The Cr cast steels (0.7 % Cr to 1.1 % Cr) are not in common use in the steel casting industry. Although Cr leads the field as an alloying element for wear-resistant steels, it is seldom used alone. For example, the Cr-Mo steels are widely used.

Cu bearing cast steels – There are several types of Cu containing steels. Selection among these different types is primarily based on either their atmospheric-corrosion resistance (weathering steels) or the age-hardening characteristics which Cu adds to steel.

High strength cast steels – These steels cover the tensile strength range of 1,200 MPa to 2,070 MPa. Cast steels with these strength levels and with considerable toughness and weldability have been originally developed for ordnance applications. These cast steels can be produced from any of the above medium-alloy compositions by heat treating with liquid-quenching techniques and low tempering temperatures.

Mechanical properties

Typical room-temperature mechanical properties of low alloy steels are a function of alloy content, heat treatment, and section size. Normally a wide range of properties are obtainable through changes in C and alloy content and heat treatment.

Physical properties

The physical properties of cast steel are generally similar to those of wrought steel.

Elastic constants – Elastic constants of C and low alloy cast steels as determined at room temperature are only slightly affected by changes in composition and structure. The modulus of elasticity ‘E’ is around 200 GPa, Poisson’s ratio is 0.3, and the modulus of rigidity is 77.2 GPa. Increasing temperature has a marked effect on the modulus of elasticity and the modulus of rigidity. A typical value of the modulus of elasticity at 200 deg C is around 193 GPa, at 360 deg C is 179 GPa, at 445 deg C is 165 GPa, and at 490 deg C is 152 GPa. Above 480 deg C, the value of the modulus of elasticity decreases rapidly.

Density – Density of cast steel is sensitive to the changes in composition, structure, and temperature. The density of medium C cast steel is around 7.8 tons/cum. The density of cast steel is also affected somewhat by mass or size of section.

Electrical properties – Electric properties of C and low alloy steel castings do not significantly affect usage. The only electrical property which can be regarded as having any importance is resistivity, which, for various annealed C steel castings with 0.07 % C to 0.2 % C, is 0.13 micro ohm meter to 0.14 micro ohm meter. Resistivity increases with C content and is around 0.2 micro ohm meter at 1 % C.

Magnetic properties – Steel castings from the housings for electrical machinery and magnetic equipment and carry only stray fluxes around the machines, hence, the magnetic properties of steel castings are less important than they have been formerly when core material was produced from commercial cast iron and steel. Low C cast electrical steel has supplanted other cast metals for housings and frames for magnetic circuits.

The C content of the steel is very important in determining the magnetic properties. The maximum permeability and the saturation magnetization decrease, and the coercive force increases, as the C content increases. Mn, P, S, and Si also increase the magnetic hysteresis loss in cast steels. Other factors being equal, the magnetic hysteresis loss is unaffected by more than 0.02 % P. Magnetic properties change considerably, depending on the mechanical treatment and heat treatment of the steel.

Cast electrical steels contain around 0.1 % C, with other alloying elements held to a minimum and the castings are furnished in the annealed condition. Specifications need 0.05 % C to 0.15 % C, 0.2 % Mn, and 0.35 % to 0.60 % or 1.50 % to 2 % Si. The maximum permeability of annealed cast electric steel which can normally be expected is 18.6 milli-Henry per meter.

As the C content is increased, maximum permeability and saturation magnetization decrease, and coercive force increases. Also, an increase in Mn and S content increases the magnetic hysteresis loss. Si and Al eliminate the allotropic transformation in iron and permit annealing at high temperature without recrystallization during cooling, thus, large grains can be achieved. These elements can be added in large quantities without affecting magnetic properties, but they do reduce the saturation value and increase the brittleness of the steel.

Hysteresis loss varies directly with grain size number, hence, the larger is the grain size, and the better is the properties. Residual alloy content is to be low since it lowers saturation value. The factors which improve the machinability of electrical steel decrease the magnetic properties. A disadvantage in the use of pure iron for the electrical steel is low resistivity. The iron is to be rolled thin to keep eddy currents at low level; otherwise, the magnetic properties are poor.

Volumetric changes – In the foundry, all volume changes of steel are pertinent, whether they occur in the liquid state, during solidification, or in the solid state. Of particular interest is the contraction which results when liquid steel solidifies. Volume changes which occur in the liquid state as the cast steel cools affect the planning for adequate metal to fill the mould. Contraction is of the order of 0.9 % per 100 deg C for a 0.3 % C steel. The exact amount of contraction varies with the chemical composition, but it is normally within the range of 0.8 % to 1 % per 100 deg C for C and low alloy steels. A larger contraction occurs upon solidification (2.2 % for nearly pure iron to 4 % for a 1 % C steel). In case of cast C and low alloy steels, a solidification contraction of 3 % is generally expected.

The greatest amount of contraction occurs as the solidified steel cools to room temperature. Solid state contraction from the solidus to room temperature varies between 6.9 % and 7.4 % as a function of the C content. Alloying elements have no significant effect on the amount of this contraction. The rigid form of the mould hinders contraction and results in the formation of stresses within the cooled casting which can be high enough to cause fracture or hot tears in the casting. The hot steel has low strength just after solidification. The rigidity of the mould makes the proper relation of casting configuration to accommodate this contraction one of the most important factors in producing a successful casting.

In commercial production, a combination of all three contraction components can operate simultaneously. Liquid steel in contact with the mould wall solidifies quickly and proceeds to solidify toward the centre of the casting. The solid envelope undergoes contraction in the solid state, while a portion of the still liquid steel is solidifying. The remaining liquid steel contracts as its temperature decreases toward the freezing point. Because of contraction factors, many casting designs need considerable development to produce a sound casting.

Engineering properties

Wear resistance – Cast steels have wear resistance comparable to that of wrought steel of similar composition and condition. Cr leads the field as an alloying element for wear resistant steels but is seldom used alone. Ni-V, Mn-Mo, and Ni-Mn cast steels are used for several structural purposes requiring wear resistance and high strength.

Corrosion resistance – Corrosion of cast steel is similar to that of wrought steel of equivalent composition. Data published on the corrosion resistance of wrought C and low alloy steels under different conditions can be applied to the cast steels. Low alloy steels are normally not considered corrosion resistant, and casting compositions are not normally selected on the basis of corrosion resistance. In some environments, however, significant differences are observed in corrosion behaviour such that the corrosion rate of one steel can be half that of another grade. In general, steels alloyed with small amounts of Cu tend to have somewhat lower corrosion rates than Cu free alloys. Even small content of Cu (0.05 %) has shown to exert a significant effect. In some environments, nominal levels of Ni, Cr, P and Si can also bring about modest improvements, but when these four elements are present, the addition of Cu holds little if any advantage.

Soil corrosion – Cast steel pipe has been tested for various periods upto 14 years in different types of soil. The results of these tests have been compared directly with results from tests on wrought steel pipe of similar composition, and no significant difference in the corrosion of the two materials have been noticed. However, the actual corrosion rate and rate of pitting of the cast pipe varied widely, depending on the soil and aeration conditions.

High temperature properties – Steels operating at temperatures above ambient are subject to failure by a number of mechanisms other than mechanical stress or impact. These include oxidation, H2 (hydrogen) damage, sulphide scaling, and carbide instability, which manifests itself as graphitization.

The environmental factors involved in high temperature service (370 deg C to 650 deg C) need that steels used in this temperature range are carefully characterized. As a consequence, four specifications have been developed for cast C and low alloy steels for high temperature service. One of these specifications consists of C steels while the other three cover low alloy steels.

The two alloying elements common to nearly all the steel compositions used at high temperatures are Mo and Cr. Mo contributes largely to the creep resistance. Depending on microstructure, it has been shown that 0.5 % Mo reduces the creep rate of steels by a factor of at least 103 at 600 deg C. Cr also reduces the creep rate, although modestly, at levels to around 2.25 %. At higher Cr levels, creep resistance is somewhat reduced. V improves creep strength and is indicated in some specifications. Other elements which improve creep resistance include W, Ti, and Nb. The effect of W is similar to that of Mo, but on a weight percent basis more W is needed in order to be equally beneficial. Ti and Nb have been shown to improve the creep properties of C free alloys, but because they remove C from the solid solution, their effect tends to be variable.

Low temperature toughness – In addition to the soundness, strength, and microstructure of a steel, toughness is also strongly affected by temperature. Steel castings suitable for low temperature service are specified in standards. Fig 4 shows the effect of temperature on the impact resistance of three grades of cast steels. Fig 4(b) also shows the effect of heat treatment on the impact resistance of Ni-Cr-Mo cast steel.

Fig 4 Effect of temperature on the Charpy V-notch energy of a C steel and two low alloy cast steels

Fig 4(a) shows Charpy V notch energies for a C cast steel (0.30 % C maximum with 1 % Mn maximum), quenched, tempered, and stress relieved, taken from a 50 mm × 230 mm × 210 mm test block and from a 75 mm × 230 mm × 283 mm test block. Fig 4(b) shows Charpy V notch energies for Ni-Cr-Mo cast steel samples (taken from 50 mm × 230 mm × 210 mm test block) from steel with two different tempering and aging treatments after being air cooled from 955 deg C, reheated to 900 deg C, and then water quenched. Fig 4(c) shows Charpy V notch energies for 2.5 % Ni cast steel samples (taken from 75 mm × 230  mm × 283 mm test block) after being air cooled (normalized) from 900 deg C and either tempered at 620 deg C or reheated to 900 deg, water quenched and then tempered at 620 deg C. All samples are taken at locations greater than one-fourth the thickness in from the surface of test blocks having an ASTM grain size of 6 to 8. The curves represent average values for several tests at each test temperature.

Fracture toughness -Temperature has an effect on the fracture toughness of a C cast steel (0.25 % C, maximum). Mn-Mo cast steels show the highest fracture toughness at both room temperature and at -45 deg C. The ferritic pearlitic cast steel shows the lowest fracture toughness at both temperatures. The martensitic cast steels (such as C-Mn, Mn-Mo, and Ni-Cr-Mo steels) have better fracture toughness at room temperature than the ferritic-pearlitic cast steels. The Ni-Cr-Mo cast steel has the largest decrease in fracture toughness at -45 deg C compared to room temperature. The C-Mn and Mn-Mo steels have ductile stable crack growth and the highest fracture toughness values at both room temperature and -45 deg C.

Machinability – Extensive lathe and drilling tests on steel castings have not revealed significant differences in the machinability of steels made by different melting processes, nor of wrought and cast steel, provided strength, hardness, and microstructure are equivalent. The skin or surface on a sand mould casting often wears down cutting tools rapidly, possibly because of adherence of abrasive mould materials to the casting. Hence, the initial cut is to be deep enough to penetrate below the skin, or the cutting speed can be reduced to 50 % of that recommended for the base steel.  Microstructure has considerable effect on the machinability of cast steels. It is sometimes possible to improve the machining characteristics of steel castings by 100 % through normalizing, normalizing and tempering, or annealing.

Weldability – Steel castings have welding characteristics comparable to those of wrought steel of the same composition, and welding these castings involves the same considerations. The severe quenching effect produced when using a small welding rod to weld a large section results in the formation of martensite in the base metal area immediately adjacent to the weld (in the heat-affected zone). This can happen even in low C steel, causing loss of ductility in the heat affected zone. Usually cast steels with a maximum of 0.2 % C and 0.5 % Mn present fewer problems from this effect. However, it is essential that all of the C steels (with more than 0.2 % C) and the air-hardening alloyed steels be preheated before welding at the standard recommended temperatures, maintaining a proper inter-pass temperature, and then post weld heat treated to produce sufficient ductility.

To prevent cracking in C and low alloy cast steels, the hardness of the weld bead is not to exceed 350 HV, except where conditions are such that only compressive forces result from the welding. This value cannot be low enough in configurations in which extreme restraint is involved. Virtually all castings receive a stress-relief heat treatment after welding, even composite fabrications in which steel castings are welded to wrought-steel shapes.

The maximum compositional limits which have been placed on readily weldable grades of castings are 0.35 % C, 0.7 % Mn, 0.3 % Cr, and 0.25 % maximum Mo plus W, with a total of 1 % undesirable elements, predicated on the widespread use of stress-relief heat treatment in the steel casting industry. For each 0.01 % decrease in the specified maximum C content, most specifications permit an increase of 0.04 % Mn above the maximum specified, upto a maximum of 1 %.

Many welds which fail do not fail in the weld but in the zone immediately adjacent to the weld. While the weld is being made, this zone is heated momentarily to a melting temperature. The temperature decreases as the distance from the weld increases. Such heating induces structural changes, specifically the development of hard, brittle areas adjacent to the weld deposits, which reduce the toughness of the area and frequently cause cracking during and after cooling. Likewise, certain alloying elements other than C, such as Ni, Mo, and Cr, bring about air hardening of the base metal. For these reasons, the quantity of alloying elements to be used is to be limited unless special precautions are taken, such as the preheating of the base metal to 150 deg C to 315 deg C. Increased hardness in the heat-affected zone of the base metal can be removed by post weld heat treating the welded casting or by heating it for definite periods at 650 deg C to 675 deg C. This also relieves stresses from welding.

For the arc welding of steel castings, a high-grade heavily coated electrode, granular flux, or CO2 atmosphere is generally desirable. These coatings contain little or no combustible material. Mineral coatings are frequently used to keep H2 absorption at a minimum level and thereby limit under-bead cracking. Selection of the number of passes and of welding conditions is similar to welding practice for wrought steels.

Welds in castings can be radio-graphed by gamma-ray or X-ray methods to ascertain the degree of homogeneity of the welded section. The most common imperfections are incomplete fusion, slag inclusions, and gas bubbles. Magnetic particle inspection is also useful in the detection of surface and near-surface cracks.

The mechanical properties of welds joining cast steel to cast steel and of welds joining cast steel to wrought steel are of the same order as similar welds joining wrought steel to wrought steel. Most tensile samples machined across the weld break outside the weld, in the heat-affected zone. This does not mean that the weld is stronger than the casting base metal. Closely controlled welding techniques and stress relieving are necessary to prevent brittleness in the heat-affected zone.

Non-destructive inspection

Highly stresses steel castings for aircraft and for high-pressure or high-temperature service are to pass rigid non-destructive inspection. Radiographic acceptance standards are to be agreed upon by the user and producer before production begins. Critical areas to be radio-graphed can be identified on the casting drawing.

Magnetic particle inspection is used on highly stressed steel castings to detect surface discontinuities or imperfections at or just below the surface. The extent of inspection and the acceptance standards are to be agreed upon by the user and producer.

Liquid penetrant inspection can be used on steel castings, but it is primarily used to inspect non-magnetic materials such as non-ferrous metals and austenitic steels for possible surface discontinuities.

Ultra-sonic testing is sometimes used on steel castings to detect imperfections below the surface in heavy sections which are 0.3 m to 8.5 m thick. Test surfaces of castings are to be free of material which can interfere with the ultra-sonic test. These castings can be as cast, blasted, ground, or machined. This technique is intended to complement the use of radiographic testing in the detection of dis-continuities.

Hydrostatic testing, or pressure testing, is used on valves and castings intended to contain steam or fluids. If a casting is to pass a pressure test, essential factors are to be noted on the blueprint, and the details of the test are to be understood by the buyer and producer.