Question

Lab Homework. Answer these questions before coming to the Lab. 1. What is the approximate amount of carbon you would want in a steel in order to be able to "quench-harden" it? 2. What happens to the crystalline (or atomie) structure when you heat the steel samples from room temperature to high temperature (austenite region)? 3. What happens to the crystalline (or atomic structure when you quench the steel samples from the high temperature (austenite region) to room temperature? 4. Based on your answer above, what do you expect to find for your hardness values for the different samples that you have quenched? 5. What is the purpose of annealing steel? 6. Based on your answer above, what effect do you expect the annealing to have on the hardness values of the steel samples?

Answer 1

Answers :- 1. Approximate amount of carbon required for steel to be quench - handen is → "0.35%to 0.60%" Quench Hardening, is the heat treatment of steel which increases its hardness by quenching. Auench hardening & tempering are confined in application to the so- called heat treatable steels it. Steels containing C in the excess of 0.35%. Cat and perhaps containing alley additions. a. The austenitizing temperature for hypocutectoid steels is usually between 723°C (1333°f) & a10°c (1670°F) & for hypereutectoid steels, austenitizing temperature is between 723°C(1333°f) and 1130 c/2086°f) When Steel sample is heated to this range from room tempereture, crystalline Structure becomes Austenite .f.c.c.( face centered catic) 8-irom for the carton percentage in between oto 2 1. Carbon. for the carbon percentage more than 27, then the crystalline structure is liquid iron and austenite. 3. Quenching is the process to harden the steel semple, when arench the steel sample from high temperature (austenite range) to room temperature, A very repid rate of cooling forces carbon to remain in solution and austenite transfromp to martensite Comparatively slower rate of cooling produces fine pearlite and stille slower cooling rate give disse to coarse pearlite. on Pearlite is harder then austenite and martensite is harder is harder then the pearlite. This accounts for the increased hardness of the quenched steel.

( 2 9. Upon quenching semples is increased Hardnes value of quenching media and Quenching Media a sted sample, & the hardness of steel to a significant value Steel sample ebtained is defendent on the rate of cooling thus associated with Hardness obtained, Re. 65 0 - Water Oil furnace 8 = air - cool 5. the process of hestate, to a templing is (usually and for i Annealing is torted studdistastion ansure structure metastable or instaliry the room tempe A purpose of Annealling Steel :- Annealing is the process of heating a metal which is in a metastable or distorted structural state, to a temperature which will remove the installity or distortion and then cooling is (usually at a slow rate) , so that the room temperature structure is stable and/or Strain free Purpose of Annealling fted are:- - Inducing a completely stable structure 9 Refining and homogenizing the structure ② Reducing Hardness | Improves machinability o Improving cold wooking, 6 Producing desired microstructure 6. Removing residual stresses & Removing gases Improving mechenical, physical, electrical & magnetic properties. 30

& Cooling associated with Annealing enables the austenite to decompose at low degrees of super cooling so ar to form → A Pearlite & ferrite structure in hypoeutectoid steels, A Pearlite + cernentite structure in bystectoid steel. After Annealing value of Hardness of specimen is 55 HRC.