Corrosion of Cast Steels...

Corrosion of Cast Steels Cast steels are generally classified into the categories of (i) carbon (C) steels, (ii) low alloy steels, (iii) corrosion resistant steels, and (iv) heat resistant steels, depending on the alloy content and the planned usage. Steel castings are categorized as corrosion resistant if they are capable of sustained operation when exposed to attack by corrosive agents at operating temperatures which are generally below 300 deg C. The high alloy iron base compositions are generally given the name ‘stainless steels’, though this name is not recognized universally. Actually, these steels are widely referred to as cast stainless steels. Some of the high alloy steels (e.g. 12 % chromium steel) show many of the familiar physical characteristics of C steels and low alloy steels, and some of their mechanical properties, such as hardness and tensile strength (TS), can be altered by suitable heat treatment. The alloy steels of higher chromium (Cr) content (20 % to 30 % Cr), Cr-Ni (nickel)  steels and Ni-Cr steels do not show the changes in phase observed in ordinary C steel when heated or cooled in the range from room temperature to the melting point. Consequently, these steels are non hardenable, and their mechanical properties depend on the composition instead of heat treatment. The high alloy steels (stainless steels) differ from C steels and low alloy steels in other respects, such as their production and properties. Special attention is required to be given to each grade with regard to casting design and casting practice in the foundry. For example, such elements as Cr, Ni, C, N2 (nitrogen), Si (silicon), Mo (molybdenum), and Nb (niobium) can exert a deep impact on the ultimate structure of these complex steels. Hence, balancing of the alloy compositions is normally required to...

Nickel in Steels

Nickel in Steels  Nickel (Ni) (atomic number 28 and atomic weight 58.69) has density of 8.902 gm/cc. Melting point of Ni is 1455 deg C and boiling point is 2910 deg C. The phase diagram of the Fe-Ni binary system is at Fig 1. Ni has a face centered cubic (f.c.c.) crystal structure. It is ferromagnetic up to 353 deg C, its curie point.   Fig 1 Fe-Ni phase diagram Ni is an important and widely used constituent of alloy steels. It is best known as a solid solution strengthener, a mild hardenability agent and, most important, as a means of promoting high toughness, especially at low temperatures. Ni is an important ingredient in stainless steel, helping it to prevent rust, scratches and resist heat. Around 65 % of global Ni production goes into the production of stainless steel. Ni alloyed steels contain as little as fraction of a percent to almost 30 % Ni. As may be expected, properties of these alloy steels range from strengths similar to plain carbon steel to some of the strongest metallic materials known. On the lower side of the Ni percentage in the steels are the alloy and HSLA (high strength and low alloy) structural steels. Hot rolled steels with yield strengths of 345 MPa may contain 0.50 % to 2.00 % Ni for toughness and added corrosion resistance. Age hardening steels contain 1.3 % to 1.5 % Ni plus copper (Cu) and niobium (Nb). Quenched and tempered or normalized and tempered structural steels contain nickel (Ni) up to 2.25 %, as well as a variety of other constituents including chromium (Cr), molybdenum (Mo) or boron (B). Nickel bearing addition agents Ni bearing addition agents are ferro- nickel (Fe- Ni) ferroalloy, Ni containing steel scrap, Nickel oxide...

Chromium in Steels

Chromium in Steels  Chromium (Cr) (atomic number 24 and atomic weight 52.01) has density of 7.1 gm/cc. Melting point of Cr is 1850 deg C and boiling point is 2680 deg C. The phase diagram of the Fe-Cr binary system is at Fig 1.  Cr has got a body centered cubic (bcc) crystal structure.   Fig 1 Fe-Cr phase diagram Around 85 % of the chromite (chrome ore) mined is used in metallurgical application, namely stainless steels, low alloy steels, high strength alloy steels, tool steels, some maraging steels (high strength alloy steels of the precipitation hardening type), and high performance alloys such as chromium-cobalt- tungsten (or molybdenum) alloys, nickel-chromium-manganese-niobium-tantalum (or titanium) alloys, nickel-chromium-molybdenum alloys, and cobalt-chromium alloys. Cr is the most versatile and widely used element in alloying of steel. It is a key component of stainless steels. Around 70 % of Cr used in steelmaking goes into the production of stainless steels. Consumption of Cr in the constructional alloy steels comes next. Most of the constructional alloy steels contain Cr less than 3 %. Tool steels, super alloys and other specialty steels, though have higher in Cr content account for lower consumption of Cr since these steels are produced in smaller quantities. Addition practice during steel making Cr in the steel comes either from Cr containing scrap or from ferrochrome (Fe- Cr) during the production of Cr alloyed steels. Fe-Cr used in steel making are commercially available in several grades . The main impurities in Fe-Cr are carbon (C) and silicon (Si). Low C grades are costlier than the high C grades. The widespread shift toward duplex refining practices such as the AOD, CLU, etc., for the production of stainless steels has resulted into the increased use of high carbon Fe-Cr. Low...

Phosphorus in Steels

Phosphorus in Steels  Phosphorus (P) (atomic number 15 and atomic weight 30.974) has density of 1.82 gm/cc. It has a melting point of 44.1 deg C and boiling point of 280 deg C. The iron (Fe) – P phase diagram is shown in Fig 1. Fig 1 Fe- P phase diagram P is normally considered an undesirable impurity in steels. It is present in varying concentrations in iron ore, is retained in hot metal, but is eliminated early in the steelmaking process. P oxidizes readily and is removed from steel as P2O5, which is taken up by the oxidizing slag, before the oxidation of carbon takes place. Carryover of any P2O5 containing oxidizing slag can result in P reversion to the steel in subsequent steelmaking operations. In normal commercial steels, residual P content is usually  at a level of 0.05 % max, but concentrations as low as 0.005 % are not unusual. P is readily removed only in basic steelmaking processes. Acidic processes must therefore begin with low P raw materials. It was the ability to remove this element that led to the widespread adoption of the steelmaking by the basic open hearth, electric arc furnace, basic Bessemer converter and subsequently basic oxygen furnace (BOF) processes. P is sometimes added intentionally to the steel to improve strength, machinability and atmospheric corrosion resistance. P is added to the steel in the form of ferro-phosphorus (Fe-P), containing 23 % to 26 % P. Fe-P fines are usually briquetted, after using a binder. Fe-P is capable of oxidizing the residual silicon to silica, thus enabling it to float out to the ladle slag during steel making. The intent is to reduce the concentration of residual siliceous inclusions, which are detrimental to machinability. Fe-P is normally added to the...

Corrosion of Steel and Corrosion Protection...

Corrosion of Steel and Corrosion Protection Corrosion is a multifaceted phenomenon that adversely affects and causes deterioration of properties in metals through oxidation. According to DIN EN ISO 8044 corrosion is defined as ‘Physical interaction between a metal and its environment which results in changes of the metal’s properties and which may lead to significant functional impairment of the metal, the environment or the technical system of which they form a part.’ Steel, the most commonly used material, corrodes in many media including most outdoor environments.  When unalloyed or alloyed steel without corrosion protection is exposed to the atmosphere, the surface takes a reddish brown colour after a short time. This reddish brown colour indicates rust is forming and the steel is corroding. While corroding the steel is getting oxidized to produce rust, which occupies approximately 6 times the volume of the original material consumed in the process. The corrosion process begins when a corrosive medium acts on the steel. The corrosion can be either chemical corrosion or electrochemical corrosion. Corrosion of steel is an electrochemical reaction that requires the presence of water (H2O), oxygen (O2) and ions such as chloride ions (Cl¯), all of which exist in the atmosphere. Atmospheric chloride ions are in greatest abundance anywhere near the coastline. This electrochemical reaction starts when atmospheric oxygen oxidizes iron in the presence of water. In addition, the atmosphere also carries emissions from human activity, such as carbon dioxide (CO2), carbon monoxide (CO), sulfur dioxide (SO2), nitrous oxide (NO2) and many other chemicals, which can also be significant in the corrosion process. The schematics of general corrosion process are illustrated below in Fig 1 Fig 1 Schematics of general corrosion process Types of Corrosion Besides general corrosion, there are various types of localized corrosion...