Steel is the most common and widely used metallic material in today’s society. It can be cast or wrought into numerous forms and can be produced with tensile strengths exceeding 5 GPa and it is highly formable.

Steel is corrosion resistant when coated with the various zinc-based coatings available today. Steel is dent resistant when compared with other materials and provides exceptional energy absorption in a vehicle collision. Also it is recycled and easily separated from other materials by a magnet.

Each chemical (e.g. carbon, manganese, silicon, nickel, chromium, molybdenum, vanadium, titanium, niobium, and aluminum) has a specific role to play in the steelmaking process or in achieving particular properties or characteristics, e.g., strength, hardness, corrosion resistance, magnetic permeability, and machinability.

Austenitic

This group of steel alloys contains chromium normally in the range 17-25% and nickel in a range 8-20%, with various additional elements to achieve the desired properties. In the fully annealed condition, the steel alloys exhibit a useful range of physical and mechanical properties. The mechanical properties can be can be increased with cold working. Welding of this group must be carried out with the correct methods but the low carbon content results in fewer problems than the Ferritic or Martensitic grades. Normally these steels are non-magnetic but when a significant amount of cold working is involved, as in centreless grinding, the magnetic permeability may be increased. If this group is included with the Ferritic and Martensitic groups it can be seen that the stainless steel alloys offer a great deal of versatility for applications within modern industry. The numbers listed below represent grades within British Standard 970(bar) and British Standard 1449 (sheet and plate). The figures in brackets after each number are the Euronorms currently being introduced to supersede British Standards.

Type 302 (BS EN 10088 1.4310)

A basic 18% chrome, 8% nickel, 18/8, grade from which the majority of other forms have been developed. It has excellent ductility and welding characteristics.

Type 304 (BS EN 10088 1.4301)

Similar to type 302 but due to lower carbon content, 0.08% is less susceptible to inter-granular corrosion after welding.

Type 304L (BS EN 10088 1.4307)

A low carbon form of 304, 0.03%-0.035% carbon maximum, designed primarily to avoid inter-granular corrosion after welding. The tensile strength is somewhat lower than type 304.

Type 321 (BS EN 10088 1.4541)

Basically type 302 but with the addition of titanium, in direct proportion to carbon content, to prevent inter-granular corrosion and offer scale resistance at higher temperatures, up to 850°C. Corrosion resistance is slightly lower than type 304. This grade is not suitable for bright or mirror polishing.

Type 347 (BS EN 10088 1.4550)

Similar to type 321 but with niobium added to stabilise the steel instead of titanium. This reduces the incidence of inter-granular corrosion, but has the effect of increasing corrosion resistance to the level of type 304.

Type 303 (BS EN 10088 1.4305)

This is a free machining variant of type 304 with added sulphur or selenium to improve machining characteristics.

Type 316 (BS EN 10088 1.4401)

This is a molybdenum bearing stainless steel designed for applications involving severe corrosion conditions, resulting in a wide application in the chemical, textile and paper industries.

Type 316L (BS EN 10088 1.4404)

Similar to type 316 but with lower carbon content, 0.03%00.035% maximum, to avoid inter-granular corrosion after welding.

Type 310 (BS EN 10088 1.4845)

A 25% chrome, 20% nickel stainless steel developed for high temperature service where high creep strength is required, its maximum service temperature is aproximately 1100°C. This group is not recommended for applications of prolonged service as brittleness may occur.

Ferritic

This group contains a minimum of 17% chrome and carbon in the range of 0.08% – 2.00%. The increase in chromium imparts increased resistance to corrosion at elevated temperatures, but the lack of mechanical properties due to the fact that it cannot be heat treated, limits its applications. Likemartensitics they are magnetic and the welding of this group should be carried out with the necessary precautions.

Type 430F (BS EN 10088 1.4105)

A 17% chrome, low alloy ferritic steel that is non-hardenable and possessing only mild cold working properties due to the high chrome content. This alloy possesses good corrosion resistance up to a temperature of approximately 800°C. It’s lack of tensile properties, and poor usability, limit its applications and as a result is usually found in strip and sheet form.

Martensitic

This group contains a minimum of 12% chrome and usually a maximum of 14% with carbon in the range of 0.08% – 2.00%. Due to the high carbon content of the steel, it responds well to heat treatment to give various mechanical strengths, such as hardness. The carbon, however, is detrimental when welding and care should be taken during this operation. In the heat-treated condition this group of steels show a useful combination of corrosion resistance and mechanical properties that qualify them for a wide range of applications. The numbers listed below represent grades within British Standard 970. The figures in brackets after each number are the Euronorms currently being introduced to supersede British Standards.

Type 410 (BS EN 10088 1.4006)

A 13% chrome, 0.5% carbon stainless alloy possessing good ductility and corrosion resistance. It can be easily forged and machined and exhibits good cold working properties.

Type 416 (BS EN 10088 1.4005)

Similar to type 410 but has added sulphur to improve usability, usually in bar form.

Type 431 (BS EN 10088 1.4057)

A 17% chrome, 2.5% nickel, 0.15% (max) carbon stainless alloy which has superior corrosion resistance to 410 or 416 due to the addition of nickel. It can be heat treated to ensure good tensile strength, in the range of 55/56 tonnes tensile. Due to its good machining properties combined with strength it has numerous applications, particularly in machined components where the above mechanical property is required. The material is usually supplied in bar form.

Effects of Alloying Elements in Steel

Steel is basically iron alloyed to carbon with certain additional elements to give the required properties to the finished melt. Listed below is a summary of the effects various alloying elements in steel.

Carbon

The basic metal, iron, is alloyed with carbon to make steel and has the effect of increasing the hardness and strength by heat treatment but the addition of carbon enables a wide range of hardness and strength.

Manganese

Manganese is added to steel to improve hot working properties and increase strength, toughness and hardenability. Manganese, like nickel, is an austenite forming element and has been used as a substitute for nickel in the A.I.S.I 200 Series of Austenitic stainless steels (e.g. A.I.S.I 202 as a substitute for A.I.S.I 304)

Chromium

Chromium is added to the steel to increase resistance to oxidation. This resistance increases as more chromium is added. ‘Stainless Steel’ has approximately 11% chromium and a very marked degree of general corrosion resistance when compared with steels with a lower percentage of chromium. When added to low alloy steels, chromium can increase the response to heat treatment, thus improving hardenability and strength.

Nickel

Nickel is added in large amounts, over about 8%, to high chromium stainless steel to form the most important class of corrosion and heat resistant steels. These are the austenitic stainless steels, typified by 18-8, where the tendency of nickel to form austenite is responsible for a great toughness and high strength at both high and low temperatures. Nickel also improves resistance to oxidation and corrosion. It increases toughness at low temperatures when added in smaller amounts to alloy steels.

Molybdenum

Molybdenum, when added to chromium-nickel austenitic steels, improves resistance to pitting corrosion especially by chlorides and sulphur chemicals. When added to low alloy steels, molybdenum improves high temperature strengths and hardness. When added to chromium steels it greatly diminishes the tendency of steels to decay in service or in heat treatment.

Titanium

The main use of titanium as an alloying element in steel is for carbide stabilisation. It combines with carbon to for titanium carbides, which are quite stable and hard to dissolve in steel, this tends to minimise the occurrence of inter-granular corrosion, as with A.I.S.I 321, when adding approximately 0.25%/0.60% titanium, the carbon combines with the titanium in preference to chromium, preventing a tie-up of corrosion resisting chromium as inter-granular carbides and the accompanying loss of corrosion resistance at the grain boundaries.

Phosphorus

Phosphorus is usually added with sulphur to improve machinability in low alloy steels, phosphorus, in small amounts, aids strength and corrosion resistance. Experimental work shows that phosphorus present in austeniticstainless steels increases strength. Phosphorus additions are known to increase the tendency to cracking during welding.

Sulphur

When added in small amounts sulphur improves machinability but does not cause hot shortness. Hot shortness is reduced by the addition of manganese, which combines with the sulphur to form manganese sulphide. As manganese sulphide has a higher melting point than iron sulphide, which would form if manganese were not present, the weak spots at the grain boundaries are greatly reduced during hot working.

Selenium

Selenium is added to improve machinability.

Niobium (Columbium)

Niobium is added to steel in order to stabilise carbon, and as such performs in the same way as described for titanium. Niobium also has the effect of strengthening steels and alloys for high temperature service.

Nitrogen

Nitrogen has the effect of increasing the austenitic stability of stainless steels and is, as in the case of nickel, an austenite forming element. Yield strength is greatly improved when nitrogen is added to austenitic stainless steels.

Silicon

Silicon is used as a deoxidising (killing) agent in the melting of steel, as a result, most steels contain a small percentage of silicon. Silicon contributes to hardening of the ferritic phase in steels and for this reason silicon killed steels are somewhat harder and stiffer than aluminium killed steels.

Cobalt

Cobalt becomes highly radioactive when exposed to the intense radiation of nuclear reactors, and as a result, any stainless steel that is in nuclear service will have a cobalt restriction, usually aproximately 0.2% maximum. This problem is emphasised because there is residual cobalt content in the nickel used in producing these steels.

Tantalum

Chemically similar to niobium and has similar effects.

Copper

Copper is normally present in stainless steels as a residual element. However it is added to a few alloys to produce precipitation hardening properties.

References:

1. Handbook of Materials Selection by MYER KUTZ

2. http://www.chasealloys.co.uk/