Thermochemical Treatment of Steel

Thermochemical treatment of steel

The thermochemical treatment is heat treatment in which , in addition to changes in the structure of the steel , are also produced changes in the chemical composition of the surface layer by adding different chemicals to a predetermined depth . These treatments require the use of controlled heating and cooling in special atmospheres . Among the most common targets of these treatments are to increase the surface hardness of the parts leaving the softer and tough core , reducing friction increasing lubricity , increased wear resistance, increase resistance to fatigue and increase corrosion resistance .

Cementation (C ): increases the surface hardness of a piece of mild steel, increasing the concentration of carbon on the surface. Is accomplished considering the environment or atmosphere surrounding the metal during the heating and cooling . Treatment results in increased carbon content in the peripheral zone , obtaining then through tempering and tempered , high surface hardness , wear resistance and good toughness in the core.

Nitridation ( N ): as the cementing surface hardness increases , but does so to a greater extent by incorporating nitrogen in the composition of the workpiece surface . By heating the steel at temperatures between 400 and 525 ° C , into a stream of ammonia gas , more nitrogen .

Cyanide (C + N): surface hardening small steel parts. Bathrooms with cyanide, sodium cyanate and carbonate are used. Temperatures between 760 apply and 950 ° C.

Carbonitriding (C + N) : as cyanidation , carbon and nitrogen introduced in a surface layer , but with hydrocarbons such as methane, ethane or propane; ammonia ( NH3) and carbon monoxide (CO ) . In the process temperature from 650 to 850 ° C are required and it is necessary to perform quenching and subsequent tempering .

Sulfinización ( S + N + C ): increases the wear resistance action of sulfur . Sulfur is incorporated into metal by heating at low temperature ( 565 ° C ) in a salt bath .

Uses: Used for smaller sprockets, scissors, high-strength screw, asparagus, guides,

cam followers, reducing axes, chisels.

Heat treatment of Steel 4140: is austeniza at temperatures between 830-850 ° C and oil quenching occurs. Tempering is given for two hours at 200 ° C to obtain hardness of 57 HRc and if it occurs at 315 ° C the hardness is 50 HRC. Annealing is heated to between 680-720 ° C for two hours maintenance, then cooled to 15 ° C per hour to 600 ° C and is terminated by cooling the still air. Strain relief is heated between 450-650 ° C and maintained between ½ and 2 hours. Cool in the oven up to 450 ° C and then allowed to cool to still air.

Corrosion fatigue behaviour of ion nitrided AISI 4140 steel.

Machine components suffer from corrosion degradation of fatigue characteristics and improvement can be attained by the application of a nitriding treatment, particularly to low alloy steels. In the present study, the effect of ion nitriding on corrosion. of AISI 4140 steel has been investigated by conducting a series of rotary bending corrosion fatigue tests at 95 Hz, in 3% NaCl aqueous solution. Hourglass shaped, 4 mm diameter fatigue specimens were ion nitrided at 748 K for 1, 3, 8 and 16 h prior to the tests. It was observed that distinct fatigue limit behaviour of ion nitrided steel in air completely disappeared in corrosive environment besides severe degradation in fatigue characteristics. An improvement reaching to 60% in corrosion fatigue strength can be attained by successive ion nitriding practice based on a fatigue life of 107 cycles. It was also determined that a power relationship holds between corrosion fatigue strength and fatigue life of ion nitrided steel. The presence of white layer has resulted in additional improvement in corrosion fatigue resistance, and it was observed that corrosion fatigue cracks were

Experimental work

A standard CrMo, medium carbon, low alloy AISI 4140 steel has been used in the study and the chemical composition of the steel is given in. The 22 mm diameter steel bar was received in hot rolled condition and austenitized at 1123