Hardening Steel Using Different Quenching Media

German – Jordanian University (GJU) Material Science Laboratory IE 2210 Summer Semester 2008‐2009

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Experiment 8

Mahmood Hisham Shubbak

Title Mark Objective /10 Introduction /15 Method /15 Result /30 Discussion and Conclusion /20 Report Neatness /10 Total Mark /100

Objective: 1. Heating steel to appropriate temperature, and then quenching it using different quenching media. 2. Measuring the hardness after quenching by different media. 3. Study the changes in hardness of steel due to the changes of the quenching media.

Introduction: Plain carbon steels and alloy steels are among relatively few engineering materials, which can be usefully heat‐treated in order to vary their mechanical properties. Steels are heat treated, because of the structural changes that can take place within solid iron carbon alloys. The various heat‐treatment process appropriate to plain carbon steels are: 1. Full annealing. 2. Normalizing. 3. Process annealing. 4. Hardening. 5. Tempering. In all above process the steel is heated slowly to the appropriate temperature according to its carbon content and then cooled.

It is the rate of cooling which determines the ultimate structure and properties that the steel will have at the end of each of the pre‐mentioned heat treatment process.

The process of hardening involves rapidly quenching the steel from appropriate temperature into quenching media. Hypereutectoid steels are heated up to 30‐50˚ C above their upper critical temperature prior to quenching. There is no particular advantage in heating hyper‐eutectoid steels above their Acm when hardening them, and in practice; the hardening temperature normally used is just 30‐50˚ C above their lower critical temperature. Quenching hypereutectoid steels from this lower temperature helps to prevent cracking and distortion.

If the steel is cooled quickly, there will be no transformation into Pearlite and ferrite or Pearlite and Cementite, but into a new harder, but less ductile structure called Martensite. This is due to the allotropic transformation of the face‐centered Austenite.

The most commonly used quenching media are: a. Compressed air blast. b. Oil. c. Water. d. 10% Brine (water + 10% salt).

In this Experiment we studied the effect of heat treatment on the properties of carbon steel especially its hardness. We used three quenching media which are air, water and inside the furnace.

Method: 1. Specimen Heating:

a) Bring three specimens with similar dimensions from steel containing more than 0.3% C bars. b) Heat the furnace to the hardening temperature of the specimen (until they become totally austenite), the hardening temperature usually depends on the percentage of carbon of the steel bar used. Then put the three specimens close to each other inside the heated furnace. In order to protect them from oxidization, it is recommended to cover them with sand inside a steel cast iron container. c) Leave the specimens inside the furnace for 30– 60 minutes.

2. Quenching in different Media:

a) Leave one specimen inside the furnace for a long time (6 hours or more); in order to cool it with small cooling rate. b) Take the second specimen out from the furnace using nickel tongs, and quench it in the air (larger cooling rate). c) Take the third specimen out, and quench it using the water media with stirring until it becomes cold. (the largest cooling rate).

3. Hardness Measuring:

a) Grind the surface of each specimen properly to prepare it for the hardness test. b) Measure the hardness for each one of the specimens using the Vickers testing machine, do 3 trials for each specimen. c) Compare between the results, and discuss the effect of each quenching medium on the hardness of the specimen.

Results:

Applying the experiment method we got the following results:

The Specimens Hardness: 1. The 1st Specimen: 9 Quenching Media: Inside the furnace (slow cooling) 9 Vickers Hardness: d1 d2 HV 1st trial 0.5388 0.5270 195.8 2nd trial 0.5471 0.5583 182.1 3rd trial 0.5546 0.5416 187.9 188.6 HV 2. The 2nd Specimen: 9 Quenching Media: air (medium cooling rate) 9 Vickers Hardness: d1 d2 HV 1st trial 0.4754 0.4456 262.3 2nd trial 0.4565 0.4518 269.9 3rd trial 0.4518 0.4628 266.0 266.1 HV 3. The 3rd Specimen: 9 Quenching Media: water (rapid cooling) 9 Vickers Hardness: d1 d2 HV 1st trial 0.3908 0.3938 361.4 2nd trial 0.4124 0.4009 336.3 3rd trial 0.4077 0.4064 341.0 346.2 HV

Discussion and Conclusion:

In this experiment we learned about Hardening Steel using different quenching media.

Heat treatment is very effective method to hardening carbon steel; the idea behind all the heat treatments is to change the structure of the alloy material in order to change its properties. In all heat treatment processes we heat the specimen to some high temperature then we cool it with some cooling rate. From the results we got in this experiment we can notice that the larger cooling rate we applied, the harder steel we got. When we use the water media (the most rapid cooling) we got very hard and brittle steel. (something like Martensite) While using the furnace cooling media, we got steel with least hardness and most ductility. (something like Pearlite) And using the air media, the output we got was something like Bainite with medium hardness and ductility (almost highest toughness). The heat treatment process (or the quenching media we used) depends always on the final product we need and its applications; for example we can use the rapid cooling process to get very hard iron and use it in gears or other high hardness needed applications.