Influence of basic components on some properties of steel

Element	Ultimate tensile strength	Yield stress	Percent elongation	Firmness	Impact elasticity	Weldability	Cold brittleness	Hot-brittleness	Corrosive resistance
Carbon	++	+	--	++	-	-	0	0	0
Manganese	+	+	-	+	0	0	0	-	+
Silicon	+	+	-	+	--	-	0	-	-
Nickel	+	+	0	+	+	0	0	0	+
Chrome	+	+	0	+	+	0	0	0	+
Copper	0	0	0	0	0	0	0	0	++
Vanadium	+	+	-	+	0	+	0	--	+
Molybdenum	+	+	-	+	0	+	0	-	+
Titanium	+	+	0	+	-	+	0	0	0
Phosphorus	++	+	-	+	-	-	++	0	+
Sulphur	-	-	0	-	-	0	0	+	0

Note: "++" - increases highly, "+" - increases, "--" - diminishes highly, "-" - diminishes, "0" - no noticeable influence .

6.4. Heat treatment and metalformimng

Heat treatment of metals. The effective method of purposeful change of the metal structure to obtain required properties is heat treatment. Heat treatment of metal materials consists in heating of them to the required temperature, curing and cooling with specified rate to the certain temperature. The processes of heat treatment are divided into heat treatment itself under the action of the heat only - heat hardening, annealing, tempering, normalization; thermo-mechanical treatment at the combined action of heat and plastic deformation; physic-thermal treatment at combination of action of heat and change of chemical composition of metal.

Heat hardening of metals consists in their heating to the temperature not below the critical points that are the points on the phase diagram, at which the phase state of alloys quality changes, their next curing and fast cooling. The heat hardening carbon and low-alloyed steels have an aim to get the needle-shaped structure of martensite. Martensite - is the oversaturated solid solution of carbon in a - Fe. At cooling in water austenit is saved to the temperature approximately 200°C, and then instantly transforms into martensit. Structure of martensite is the most solid and brittle structure of steel.

To remove the internal stresses which arise up at heat hardening of alloys, and achievement of the best combination of the strength and plasticity they are subjected to tempering - that is to the heating till temperature below the lower critical points. There are distinguished low, middle and high tempering.

Temperature of heating at low tempering is 150-200°C, and at high tempering – 600-650°C. During the tempering of steel the martensit transforms into more stable structures.

If it is required to reduce brittleness, to increase plasticity and viscidity of metals, their workability, annealing is applied. The characteristic feature of annealing is slow cooling, which is achieved under the layer of sand, ash, slag etc. At annealing of steels after the previous heating austenite structure forms, which slowly cools down, transforms to the equilibrium structure in accordance with the phase diagram.

For achievement of fine-grained homogeneous structure with lower plasticity, but with higher brittleness, than after annealing, normalization of alloys is conducted, the peculiarity of which is air cooling. Normalization is more simple type of treatment comparing with annealing. It promotes brittleness and enables to get the cleaner surface of steel at cutting.

The thermomechanical treatment of metals includes heating, plastic deformation and cooling of metal, combined in the single technological process. The essence of combined process consists in the hardening of part blanks just after the end of hot-plastic working (forging, rolling). At the same time part blanks are specially not heated, but remaining heat is utilized after a hot molding. Due to that fuel is saved for heating at hardening, demand in heater furnaces diminishes, time on making the details shortens and mechanical properties are substantially improved.

The chemical thermal treatment of metals consists in the saturation of products surfaces with the carbon, nitrogen, aluminum and other elements. Products, assigned for the high weathering in combination with the shock loadings, are subjected to this type of treatment. Such wares must have high hardness of surface coatings and enough viscid core. Depending on the peculiarities of the deformed product it is possible to influence also on the fatigue strength, to promote resistance of material surface to the action of external aggressive environments etc.

More frequently cementation - saturation of steel with a carbon - is used. Surface coating of low-carbon steels during the cementation carbonizes to 0.8-1.1% C and is subjected to the heat treatment, resulting martensite structure forming. At the carbonization the products are placed in the steel boxes, filled with the grout mixture which consists of absorbent carbon and carbonate, heated to the temperature 900-950°C. Oxygen of air, heating with carbon, forms the carbon oxide which in the case of presence of iron dissociates with formation of carbon.

The nitriding, cyanidation, aluminizing, chromium coating, and other types of the chemicothermal treatment of metals are used except of the cementation.

The surface treatment of products by the optical quantum generators (lasers) spreads at the last years. Comparatively with other kinds of treatments the laser heating has such advantages: possibility of treatment of the out-of-the-way places and surfaces of details of the complicated configuration; absence of warping and deformation of details; possibility of ray energy passing on the large distances and treatment of details, making from any materials (irons and steels, non-ferrous and solid alloys, powdered metals); high speed of the process and others like that. Application of modern laser devices provides possibility of complete mechanization and automation of the process.

Metalforming. Metalforming is carried out by rolling, dragging, pressing, forging and punching. These methods are based on the use of plastic deformation of the cold or heated metal under the action of rollers, stamps, firing-pins and others.

N ot only their form but also structure changes at the plastic deformation of metals - there is stretching of grains, grinding of them. Treatment of metals in the cold state results in peening - the increasing of ultimate strength and hardness and the declining of ductile and shock viscidity. The temperature considerably influences on the strain hardening. If temperature rises, ductility grows continuously, and deformation resistance diminishes. Within reasonable limits of heating there is the recrystallization, the previous structure of material recommences. The deformation resistance of metals grows if the speed of deformation increases.

Rolling - metalforming, at which the deformation is carried out with squeezing between the cylinders (rollers) of rolling mill, which are revolved (Fig. 6.10, a). The initial materials for the production of rolled products are casting blocks, and final products are various sections.

Dragging is a treatment of metals by pressure by pulling of wire, stick or pipe through the opening of matrix with a section, smaller than initial section of blank. The products obtain the specified geometrical shape, exact sizes, clean surface by dragging. Dragging of metals is carried out mainly in the cold state in the special drawbenches (Fig. 6.10).

The compacting (pressing) is punching shear of ductile materials through the opening of matrix. This method of ductile working is applied mainly for manufacturing products of various types from the nonferrous metals and in some cases also from steel.

Forging and punching are processes of plastic working of metals, executable on the special equipment — hammers with mass of falling parts to 5000 kg or hydraulic presses, which developing efforts to 200 mN. Forging is produced at heating of metal to the so-called forging temperature with the purpose of increase of its ductility and decline of resistance to deformation. The temperature interval of forging depends on chemical composition and structure of the processed metal, and also from the type of technological operation. For steel temperature interval 800-1100°C and for aluminum alloys — 420-480 °C. At punching a metal is limited from every quarter by the walls of stamp. During deformation it takes the shape of this cavity.