Heat resisting steels
Heat resisting steels: Heat resisting steels are practically suitable for working at even very high temperatures. Such steels must resist the influences which lead to failure of ordinary steels when put to work under high temperatures. Alloy steel containing 23-30% chromium with less than 0.35% C are mainly used to impart heat resisting service in the temperature range between 815- 1150 °C. The furnace parts and annealing boxes are generally made by this steel. These steels are particularly suitable for working at high temperatures and are thus stable at high temperatures. A steel containing chromium, nickel and tungsten, with the carbon content suitably controlled provide useful combination of non-scaling and strength retaining properties at high temperature. Such steels can work satisfactory up to 700°C and contains 0.15% C, 0.5 to 2 % Si, 0.5% Mn, 1.0 to 6%, Cr and 0.5%. Mo.
Applications:These are used in nuclear power plant, furnaces, supersonic aircrafts, missiles, annealing boxes etc.
Spring steels: Spring steels are used for the making springs. Various types of these steel along with their composition and uses are discussed as under.
(i) Carbon-manganese spring steels. This type of steel contains
C = 0.45 to 0.6, Si = 0.1 to 0.35% and Mn = 0.5 to 1.0%.
These steels are quenched and tempered up to 350 BHN. They are widely used for laminated springs for railway and general purposes.
(ii) Hyper-eutectoid spring steels. This type of steel contains
C = 0.9 to 1.2%, 0.3% (max) and Mn = 0.45 to 0.70%.
These steels are oil quenched and tempered at low temperature. This type of steel is used for volute and helical springs.
(iii) Silicon-manganese spring steels. This type of steel contains
C = 0.3 to 0.62%, Si = 1.5 to 2% and Mn = 0.6 to 1 %.
These steels are hardened and tempered. This type of steel is used for the manufacturing of railway and road springs generally.
Structural steels: Structural steels possess high strength and toughness, resistance to softening at elevated temperatures and enough resistance to corrosion. In addition, they should possess weldability, workability and high hardenability. The principal alloying elements in structural steels are chromium, nickel and manganese. These steels has various applications which are given as under:
Applications: They are used for structural members of bridges, buildings, rail road, cars etc. They are also used for manufacturing components subjected to static and dynamic loads. These components include valves, pins, studs, gears, clutches, bushes, shafts etc.
stainless steel: Stainless steel contains chromium together with nickel as alloy and rest is iron. It has been defined as that steel which when correctly heat treated and finished, resists oxidation and corrosive attack from most corrosive media. Stainless steel surface is responsible for corrosion resistance. Minimum chromium content of 12% is required for the film’s formation, and 18% is sufficient to resist the most severe atmospheric corrosive conditions. Their principal alloying element is chromium while some other elements like nickel, manganese etc. can also
be present in small amounts. Addition of nickel improves ductility and imparts strength. Corrosion resistance to stainless steels increases with increase in nickel content against neutral chloride solution and weakly oxidizing acids. Addition of molybdenum improves its resistance to sulphuric, sulphurous and organic acids. Addition of manganese increases hot workability of these steels.
Steels having 15 to 20% Ni and about 0.1 % carbon possesses great strength and toughness and extremely good resistance to corrosion. Such steels are called stainless steels. Another type of stainless steel containing 11 to 14% chromium and about 0.35% carbon is used for cutlery,
surgical and dental instruments and other purposes where hard edges are required. Maximum resistance to corrosion is obtained when this steel is ground and polished after heat-treating.
A steel containing 18% chromium and 8% nickel is widely used and is commonly referred to as 18/8 steel. Stainless steel is highly resistance to corrosion and oxidation. It can be classified into three major categories according to the type of micro structures.
General Properties of Stainless Steels It possesses wide range of strength and hardness, high ductility, formability, high corrosion resistance, good creep resistance, good thermal conductivity, good machinability, good weldability, high hot, cold workability, high resistance to scaling and oxidation at elevated temperatures, excellent surface appearance and finish.
Classification of Stainless Steel: On basis of their structure, stainless steels are classified as follow:
- Martensitic stainless steels
- Ferritic stainless steels
- Austenitic stainless steels.
These types of stainless steel are discussed as under.
Martensitic Stainless SteelsThese steels contain 12 to 16% chromium and 0.1 to 1.2 per cent carbon. The structure consists of hard martensite phase after hardening. The general utility chromium stainless steel with 12% chromium and 0.15% carbon are ferromagnetic and air hardening. It is very hard and possesses high strain and high corrosion resistance properties.
Applications : Stainless steels containing 12 to 14% chromium and 0.3% carbon are extensively used for table cutlery, tools and equipments etc. Stainless steels containing 16-18% chromium and 0.2% carbon are used as springs, ball bearing, valves, knife blades and instruments under high temperature and corrosive conditions. These steels are generally used for making utensils, surgical and dental instruments, and springs of high temperature operations, ball valves and toilet seats.
Ferritic Stainless Steels: Ferritic stainless steels are non hardenable and contain 16 to 30% chromium and 0.08 to 0.2 per cent carbon. Structure of these steel consists of ferrite phase which cannot be hardened by heat treatment. They have very low carbon and possess considerable ductility, ability to be worked hot or cold, excellent corrosion resistance and are relatively in expensive. They are always magnetic and retain their basic microstructure up to the melting point.
Applications:These are extensively used for kitchen equipment, diary machinery interior decorative work, automobile trimmings, chemical engineering industry, stainless steel sinks, food containers, refrigerator parts, beer barrels, automobile trimming etc. These are also used as
high temperature furnace parts when chromium content is high. Austenitic Stainless Steel: Addition of substantial quantities of Ni to high Cr alloys gives rise to, austenitic steel. It has good resistance to many acids (even hot or cold nitric acid). Slight amount of W and Mo are added in such steels to increase its strength at elevated temperatures. This steel contains 16 to 24% Cr, 8 to 22% Ni and less than 0.2% C. Addition of nickel stabilizes austenite, and hence the structure of these steels consists of austenite at room temperature. A steel containing 18% Cr and 8% Ni is very widely used and is commonly referred to as 18/
8 stainless steel. These steels do not harden by heat treatment but can be rolled hard. These steels possess a brilliant luster when polished. These are highly resistant to many acids even nitric acids.