Alloying Elements of Stainless Steels and Their Metallurgical Effects
Each of the alloying elements has a certain effect on the properties of the steel. It is the combined effect of all alloying elements and to some extent impurities that determine the property profile of a particular steel grade. To understand why different grades have different compounds, the alloying elements and their effects on the structures and properties should be known. Attention must be paid that the effect of alloying elements varies between hardenable and non-hardenable stainless steel in some aspects.
It is found in the material in ratios varying between 0.025-0.040% and it is one of the core alloying elements available in the stainless steel structure. The reason why these ratios are low is to prevent the precipitation of chromium carbide and to give the steel its stainless property. Carbon is a strong austenite former and it supports a strong austenitic structure. It also significantly increases the mechanical strength. Carbon reduces resistance to intergranular corrosion.
In ferritic stainless steels, carbon significantly reduces both toughness and corrosion resistance.
It increases carbon hardness and strength in martensitic and martensitic-austenitic steels. An increase in hardness and strength in martensitic steels is often accompanied by a decrease in toughness, and thus, carbon reduces the toughness of stainless steels.
Chromium is the most important alloying element in stainless steel. It is the most important alloying element that provides the corrosion resistance of stainless steels with the chromium-rich (Fe, Cr)2O3 layer that it forms on the material surface. It is the element that gives the basic corrosion resistance of stainless steel. Corrosion resistance and scaling resistance increase with increasing chromium content. It also enhances resistance to oxidation at high temperatures. Chromium supports a ferritic structure.
Nitrogen is a very strong austenite former and it supports a strong austenitic structure. It also significantly enhances mechanical strength. Nitrogen increases resistance to local corrosion, specifically in combination with molybdenum. It is used to prevent grain growth at high temperatures in high chromium and low carbon steels. It reduces the weld metal toughness at low temperatures.
The main reason for nickel addition is to promote an austenitic structure. Nickel generally increases ductility and toughness. It also reduces the corrosion rate and thus, it is advantageous in acid environments. In precipitation hardening steels, nickel is also used to form intermetallic compounds used to increase strength. It has the effect of increasing the weld mental toughness.
Copper increases corrosion resistance in certain acids and promotes an austenitic structure. In precipitation hardening steels, copper is used to forming intermetallic compounds used to increase strength
Manganese is often used to enhance hot ductility in stainless steel. Its effect on the ferrite/austenite balance varies with temperature: At low temperatures, manganese is an austenite stabilizer, but it stabilizes ferrite at high temperatures. Manganese increases the nitrogen solubility and it is used to achieve high nitrogen content in austenitic steels.
Titanium is a strong ferrite former and a strong carbide former, thus it lowers the effective carbon content and promotes a ferritic structure in two ways. It is added to increase resistance to intergranular corrosion in austenitic steels and it also increases mechanical properties at high temperatures. It is used as a balancing element to prevent chromium-carbide precipitation in austenitic stainless steel.
In ferritic stainless steels, titanium is added to increase toughness and corrosion resistance by reducing the amount of intermediate in solid solution.
In martensitic steels, titanium reduces the martensite hardness and increases the tempering resistance. In precipitation hardening steels, titanium is used to form intermetallic compounds used to increase strength. It is added with aluminum (Al) to affect the age hardening of high strength and heat-resistant alloys.
Niobium is both a strong ferrite and carbide former. It is a moderate ferrite former like titanium. In austenitic steels, it is added to enhance the resistance to intergranular corrosion and used to counteract chromium carbide precipitation, but it also enhances mechanical properties at elevated temperatures.
In martensitic steels, niobium reduces hardness and increases tempering resistance. In some types of martensitic stainless types, it is added to reduce the hardening tendency of the steel by bonding the carbon. It is added to some high-strength alloys to affect hardness and strength. In the USA, it is also referred to as Columbium (Cb).
Silicon increases resistance to oxidation in strong oxidizing solutions at both higher temperatures and lower temperatures. It is a moderate ferrite builder. It increases the tensile strength and proportionality limit in quenched steels. It reduces the ability of cold forming. It increases the electrical resistance of steel.
Molybdenum substantially increases resistance to both general and local corrosion. It is used to increase the overall corrosion resistance in non-oxidizing environments and pitting corrosion resistance in other environments. It slightly increases the mechanical strength and strongly supports a ferritic structure. Molybdenum also promotes the formation of secondary phases in ferritic, ferritic-austenitic, and austenitic steels. It will increase hardness at higher tempering temperatures due to its effect on carbide precipitation in martensitic steels. It increases strength and creep resistance at high temperatures.
It is a strong ferrite former. Aluminum increases oxidation resistance if it is added in significant quantities. It is used in some heat-resistant alloys for this purpose. In precipitation hardening steels, aluminum is used to form intermetallic compounds that increase strength in aged condition. It reduces the effect of age hardening by adding to some high-strength alloys with titanium. By adding to the filler metal containing 12% chromium, it makes the structure ferritic, in other words not non-hardenable.
Cobalt is only used as an alloying element in martensitic steels, where it increases hardness and tempering resistance, specifically at high temperatures. It is also added to improve the creep and strength properties of many stainless alloys at high temperatures.
Vanadium increases the hardness of martensitic steels due to its effect on the type of carbide available. It also increases the annealing resistance. Vanadium stabilizes the ferrite and promotes high content of ferrite in structure. It is only used on hardenable stainless steel.
Sulfur is added to some stainless steel to enhance machinability and susceptibility to machining. Excess sulfur will significantly reduce corrosion resistance, ductility, and manufacturing properties such as weldability and formability.
Cerium is one of the rare earth metals and it is added in small quantities to some heat resistant temperature steels and alloys to enhance the resistance to oxidation and high-temperature corrosion.
Tungsten is a strong ferrite former. It is added to enhance the strength and creep resistance of some high-temperature alloys.
One of these elements is added to the stainless steel with a small amount of molybdenum or zirconium, increasing the susceptibility of stainless steel to machining. These three elements promote cracking in the weld metal.
Physical and Mechanical Properties of Stainless Steels
In terms of physical properties, stainless steels vary distinctly from carbon steel in some aspects. There are also notable differences between the various stainless-steel categories. The table below shows typical values for some physical properties of stainless steel.
| Propery | Type of Stainless Steel | |||
| Martenistic | Ferritic | Austenitic | Duplex | |
| Density (g/cm3) | 7.6-7.7 | 7.6-7.8 | 7.9-8.2 | 8 |
| Young’s Modulus (MPa) | 220 | 220 | 195 | 200 |
| Thermal Expansion (x10-6/°C) | 12-13 | 12-13 | 17-19 | 13 |
| Thermal Conductivity (W/m°C) | 22-24 | 20-23 | 12-15 | 20 |
| Heat Capacity (J/kg°C) | 460 | 460 | 440 | 400 |
| Resistance (nΩm) | 600 | 600-750 | 850 | 700-850 |
| Ferromagnetic | Yes | Yes | No | Yes |
Austenitic steels generally have a higher density than other types of stainless steel. Within each steel category, the density generally increases with increasing alloying elements, specifically with heavy elements such as molybdenum.
Two important physical properties that show the greatest difference between types of stainless steel and also differ significantly for stainless steel and carbon steels are thermal expansion and thermal conductivity. Austenitic steels exhibit significantly higher thermal expansion than other types of stainless steels. This can lead to thermal stresses in applications with temperature fluctuations in the heat treatment of all structures and welding. The thermal conductivity for stainless steel is generally lower than for carbon steel and it decreases with increasing alloy level for each stainless steel category. Thermal conductivity decreases in the following order: martensitic steels, ferritic and ferritic-austenitic steels, and finally austenitic steels having the lowest thermal conductivity.
The physical properties of stainless steel are quite different from commonly used non-ferrous alloys such as aluminum and copper alloys. However, in comparison to the various families of stainless with carbon steel, many similarities exist in properties despite some fundamental differences. Like carbon steels, the density of stainless steels is approximately ~8.0 g/cm3. It is three times larger than aluminum alloys (2.7 g / cm3). Like carbon steels, stainless steels have a high elastic modulus (200 MPa or 30 KSI), which is nearly two times that of copper alloys (115 MPa or 17 KSI) and almost three times that of aluminum alloys (70 MPa or 10 KSI).
Differences between these materials are also clear in terms of thermal conductivity, thermal expansion, and electrical resistance. It greatly varies in thermal conductivity between various types of materials; 6061 aluminum alloy (Al-1Mg-0.6Si-0.3Cu-0.2Cr) has a very high thermal conductivity, and this is followed by aluminum bronze (Cu-5Al), 1080 carbon steel, and then stainless steels. For stainless steels, alloy additions, specifically nickel, copper, and chromium greatly reduce thermal conductivity.
Thermal expansion is greatest for 6061 type aluminum alloy and this is followed by aluminum-bronze and austenitic stainless alloys and then, ferritic and martensitic alloys. Additions of nickel and copper for austenitic stainless alloys may reduce thermal expansion
Stainless steels have high electrical resistance. Alloy additions tend to raise electrical resistance. Therefore, ferritic and martensitic stainless steels have lower electrical resistance than austenitic, duplex, and PH alloys, but higher electrical resistance than 1080 carbon steel. The electrical resistance of stainless steel is ~7.5 times higher than aluminum bronze and approximately 20 times higher than 6061 type aluminum alloy.
Stainless Steel Standards
Stainless steel is one of today’s indispensable industrial materials due to its resistance to corrosion, capability to be used at low and high temperatures, mechanical properties, ease of production, hygienic and long life.
**Click on the table to enlarge.
