Heat Resistant Steels
Heat Resistant Steels
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 exposure to hot gases and combustion products at temperature higher than 500 deg C. These steels are solid solution strengthened alloy steels. As these steels are used over a certain broad temperature ranges, these steels are usually 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.
The level of the heat resistance of the heat resistant steels depends on the environment conditions in which they operate and cannot be characterized by a single testing method. Maximum service temperatures which can be extended to 1150 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 molten metal and slag is also limited in these steels.
In heat resistant steels, the two most important elements are chromium for oxidation resistance and nickel for strength and ductility. Other elements are added to improve these 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. Chromium is the element which makes the micro structure ferritic.
- Nickel – Nickel when added to the heat resistant steels increases its ductility, high temperature strength and resistance to both carburizing and nitriding. Nickel tends to make the atomic structure austenitic. It decreases the solubility of both carbon and nitrogen in austenite.
- Carbon – Carbon is the most important strengthening element. Carbon is controlled within certain limits in heat resistant steels. Most heat resistant steels contain 0.05 % to 0.10 % of carbon. Cast heat resistant steels have usually 0.35 % to 0.75 % of carbon. Carbon dissolves in alloy and induces solution strength. It is also present as small, hard particles called carbides which are chemical compounds of carbon with metallic elements such as chromium, mollybdenum,titanium and niobium etc.
- Nitrogen – Nitrogen is present in heat resistant steels in small amounts and serves to strengthen both martensitic and austenitic steels.
- Silicon – Silicon decreases the solubility of carbon in the metal, which is an important variable in the steel making process. It is a strengthening element normally above 0.04 %. Silicon improves oxidation and carburization resistance, as well as resistance to absorbing nitrogen for heat resistant steels at high temperature.
- Sulphur – It is regarded as impurity and is commonly specified as upper limit in the heat resistant steels. Sulphur is detrimental to weldability but it improves machinability.
- Phosphorus – Phosphorus is usually an undesirable element in heat resistant steels since it has brittle effect when it segregates at the grain battery. It is also harmful to nickel alloy weldability. It is normally specified as upper limit for most of the heat resistant steels.
- Other alloying elements – Other alloying elements used in the heat resistant steels are manganese, molybdenum, titanium, vanadium, tungsten, aluminum, cobalt, niobium, zirconium, copper, and the rare earth elements like boron, cerium, lanthanum and yttrium. These elements improve the steels integrative properties at elevated temperature. While some elements are used for strength others are used mostly for oxidation resistance, process workability and microstructure stability.
Generally there are two fundamental classes of heat resistant steels. These are given below:
Ferritic/martensitic heat resistant steels
These steels have the same body centered cubic crystal structure (Fig 1) as that of iron. These steels consist basically of iron with small percentage of alloying elements. The main alloying element is chromium with percentage ranging from 2 % to 13 %. These steels also contain small percentages of carbon, silicon, manganese, molybdenum, aluminum and nitrogen. These elements help in precipitation hardening which supports the high temperature behaviour of the steel. Ferritic steels have transformation free ferritic structure. These steels display relatively low toughness under impact loading. Above 900 deg C these steels suffer grain coarsening combined with embrittlement. Ferritic steels are difficult to form and hence should only be welded by arc welding. The steels are insensitive to sulphurous gases. Ferritic grades are more popular heat resistance steels since they are economical because of lower alloying elements in them. These are also called low alloy heat resistant steels. Besides chromium, some of the alloying elements present in the ferritic grades are molybdenum, tungsten, niobium, vanadium, boron and titanium etc. The oxidation resistance of these steels at red hot conditions is in direct proportion to the chromium content of the steel. Ferritic/martensitic steels used for high temperature service can be classified into two categories based on the content of alloying elements and the microstructures. The first category of these steels are called low alloy steels having 1 % to 3 % chromium in them and with total alloying elements of less than 5 %. The second category of these steels is martensitic heat resistant steels. These steels include medium chrome steels with a chromium content of 5 % to 9 % and high chrome steels with a chromium content of 9% to 12 %. The total alloying elements in these steels ranges from 10 % to 20 %. High chrome steels have better creep strength.
Austenitic heat resistant steels
When sufficient nickel (more than 8 %) is added to the iron chromium steels, the steel structure becomes transformation free austenitic structure which has a face centered cubic crystal structure (Fig 1). Austenitic steels 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 generally 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 more expensive because of their higher alloy content.
Fig 1 BCC and FCC structure
Some important points related to heat resistant steel
Selection of heat resistant steel 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 alloyed and hence more heat resistant may be disadvantageous because of embrittlement besides having a higher cost. Heat resistant steel must not be exposed to flame and a direct contact with carbon must be avoided to prevent the lowering of heat resistance due to carburization.
Heat resistant steels are used in industrial furnaces, steam boilers, steam tubes, recuperators, chemical and petroleum industries, gas and fuel lines, fire boxes, heaters, resistors, heat exchangers and waste incineration plants etc.