Hardening process

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1. SUBJECT:- MATERIAL SCIENCE & METALLURGY TOPIC :- HARDENING PROCESS INSTITUTE OF TECHNOLOGY & MANAGEMENT http://alltypeim.blogspot.in/

2. WHAT IS HARD OR HARDNESS??? The ability of a material to withstand scratching, wear & penetration but harder bodies is known as “HARDNESS”. HARDNESS is dependent on ductility, plasticity, strain, strength & viscosity. HARDNESS is measured by determining the resistance to penetration by different methods such as “ROCKWELL HARDNESS TEST”. Some e.g. of hardness are Ceramics, concrete, certain metals & super hard materials. http://alltypeim.blogspot.in/

3. HARDENING High hardness values can be obtained but the process of “ HARDENING ”. Hardening consists of “heating to hardening temperature, holding at that temperature”, followed by “ RAPID COOLING ” such as quenching in water, oil or salt bath. High hardness is obtained due to phase transformation at considerably low temperature in non equilibrium phase namely “ Martensite ”. It is also called as “ Hardening by quenching ”. http://alltypeim.blogspot.in/

4. PURPOSES OF HARDENING Main purpose to develop “ High Hardness ” To improve wear resistance for tool steel To improve Tensile strength for structural steel Improve Yield strength spring steel http://alltypeim.blogspot.in/

5. HARDENING TEMPERATURES http://alltypeim.blogspot.in/

6. FACTORS AFFECTING HARDENING PROCESS The properties of hardened steel depends upon the following factors :- (a) Chemical composition (b) Size & shape of steel part (c) Hardening cycle (d) Homogeneity & grain size of Austenite (e) Quenching media (f) Surface condition of steel part http://alltypeim.blogspot.in/

7. Chemical composition :- It determines hardening temperature since it decides critical temperatures. Alloying elements, their nature & amount are very important. The Austenite stabilizer results in high amount of RA . Carbide former improves wear resistance of steel. Size & shape of steel part :- Larger parts with thickness heated slowly. Holding time must be sufficient to ensure uniform temperature throughout the specimen. This results in production of high magnitude of internal stresses due to volume change . http://alltypeim.blogspot.in/

8. Hardening cycle :- Hardening temperature should be such that homogeneous austenite with minimum grain size is produced. The cooling rate must be just equal to CCR. Slow heating rate are to be avoided in case where the problem of oxidation & decarburization is encountered. Homogeneity & grain size of austenite :- The properties of steel largely depends upon the nature of austenite since martensite directly formed from austenite . Coarse grains are not desirable. http://alltypeim.blogspot.in/

9. Quenching media :- Quenching media must provides the cooling rate higher than CCR . The quenchant properties such as thermal conductivity, latent heat of vapourization , specific heat, its temperature affects cooling rate. Surface condition of steel part :- presence of oil, greases, scales & other particles on steel surface may leads to incomplete hardening. http://alltypeim.blogspot.in/

10. HARDENING OR QUENCHING METHODS Hardening can be done in many ways, depending upon quenching procedures. The different methods are :- A. Conventional quenching B. Quenching in stages in sequence C. Spray quenching D. Quenching with self tempering http://alltypeim.blogspot.in/

11. Conventional or direct quenching :- Simplest & extensively used method. The component is quenched from hardening temperature in quenching bath & allowed too cool up to temperature of quenching bath. Sever internal stresses, distortion & cracking are developed due to drastic cooling . Suitable for only smaller sections & simple sections. Quenching in stages :- It consists of quenching the steel parts from hardening temperature to a bath maintain at a predetermined temperature higher than Ms temperature . The is then quickly transfer to a milder quenchan to cool down to room temperature. http://alltypeim.blogspot.in/

12. Quenching with self-tempering :- Here the part is quenched from hardening temperature & is withdrawn from quenching bath after some time without allowing it cools completely in the quenching medium. i.e considerable amount of heat will retained in the central portion . Then component is cooled to room temperature in a mild coolant like oil or air, first cooling forms martensite up to certain extent and second cooling undergo austenite to pearlite transformation and tempering of martensite formed in first stage . Process result in tough core & hard surface. Additional tempering is not required . http://alltypeim.blogspot.in/

13. Spray quenching :- The cooling from hardening temperature is done by spraying quenchant contiuously on steel part. The heat extraction rate is higher than direct quenching as quenchant is always in contact with the steel surface. There is no formation of vapour film over the steel surface. Suitable for selective hardening. http://alltypeim.blogspot.in/

14. APPLICATIONS Construction materials – High strength reduces the need for the material thickness which generally saves weight& cost. Machine cutting tools (drill bits, taps, laths tools) needs be much harder. Knife blades – a high hardness blade keeps a sharp edge. Bearings – necessary to have a very hard surface that will withstand continued stresses. Armor plating – High strength is extremely important both for bullet proof plates & for heavy duty containers for mining & construction. http://alltypeim.blogspot.in/

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