Heat treatment of mild and stainless steel

[ballballisicious]

is tempering the correct heat treatment for automotive panel? coz I've seen different version of heat treatment.. some books said tempering, website said (key to steel) said case processing/ annealing (seldom quenching and tempering), actually which one is correct?
actually i'd prefer tempering, coz that's what i got from the sch. library bk,

and for stainless steel, i believe it'll be tempering as well, rite?

it'll be really great if someone can specify the temperature of mild steel needed in heat treating mild steel for automotive.. coz there's so many version in the web and books, i don't want to sound like contradicting myself

 

[Astronuc]

Annealing, Tempering

Do you have particular grades of mild steel and stainless steel in mind?

Try this site for some different grades with some options for heat treatment including both annealing and tempering.

http://www.suppliersonline.com/Rese...FamilyID=4&Chemical=1&Physical=1&Mechanical=1

 

[ballballisicious]

no i dun have a particular type, I'm talking about generally what method will be used

 

[Astronuc]

Here are some discussions of annealing and tempering of steel, including mild steels and stainless steels. The annealing and tempering schedules depend on many variables including composition, cold-working schedule, etc. Its best to find a particular grade and look as the heat treatment for that grade.

http://www.anvilfire.com/FAQs/heat_faq_index.htm - discussion on heat treating of different steels.

"To harden most steel it is heated to a medium red or slightly above the point where it becomes non-magnetic. It is then quenched in water, oil or air depending on the type of steel. The steel is now at its maximum hardness but is very brittle. To reduce the brittleness the metal is tempered by heating it to some where between 350°F and 1350°F. This reduced the hardness a little and the brittelness a lot. Most steels need to be tempered at about 450°F for maximum usable hardness but every steel is slightly different.

To soften steel so that it can be cold worked and machined is called annealing. To anneal steel is is heated to slightly above the hardening temperature and then cooled as slow as possible. Cooling is done in an insulating medium such as dry powdered lime or in vermiculite. High carbon and many alloy steels can only be cooled slow enough in a temperatue controlled furnace since the cooling rate must be only 20 degrees F per hour for several hours."


http://www.azom.com/details.asp?ArticleID=543 - General article on heat treating of steel.

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http://www.key-to-steel.com/Articles/Art15.htm

It is not always necessary to heat the steel into the critical range. Mild steel products which have to be repeatedly cold worked in the processes of manufacture are softened by annealing at 500° to 650°C for several hours. This is known as "process" or "close" annealing, and is commonly employed for wire and sheets. The recrystallisation temperature of pure iron is in the region of 500°C consequently the higher temperature of 650°C brings about rapid recrystallisation of the distorted ferrite Since mild steel contains only a small volume of strained pearlite a high degree of softening is induced. As shown, Fig. 1b illustrates the structure formed consisting of the polyhedral ferrite with elongated pearlite (see also Fig. 2).

Prolonged annealing induces greater ductility at the expense of strength, owing to the tendency of the cementite in the strained pearlite to "ball-up" or spheroidise, as illustrated in Fig. 1c. This is known as "divorced pearlite". The ferrite grains also become larger, particularly if the metal has been cold worked a critical amount. A serious embrittlement sometimes arises after prolonged treatment owing to the formation of cementitic films at the ferrite boundaries. With severe forming operations, cracks are liable to start at these cementite membranes.

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http://www.askzn.co.za/tech/tech_grade_3cr12.htm
- 12% Cr steel and stainless steels properties

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See also
http://www.key-to-steel.com/Articles.asp?PageIndex=12
http://www.key-to-steel.com/Articles.asp?PageIndex=13

for some articles on stainless steels and tempering of steels.

 

[ballballisicious]

for martensite formed in quenched steel, the carnon atoms are found in the interstitial places of the bcc iron unit, so create a lot of strain to the lattice, making it deform and form a tetragonal structure, this it what i get from some website

''All atoms shift in concert, and no individual atom departs more than a fraction from its previous neighbor. Being diffusionless, the change is very rapid. All of the carbon that was present remains in solid solution and does not have a chance to form carbides. The resulting body-centered structure is Tetragonal (bct) rather than cubic. ''
i don't understand why it lead to a shearing action and why is it create a lot of strain making the steel hard and brittle

 

[Astronuc]

The martensitic (crystal, or lattice) structure has a greater specific volume than the fcc (austenitic) lattice, with c/a ration of \sqrt{2}\,/\,1.

In the bct lattice, the carbon atoms (in solid solution) cause displacement of the Fe atoms from their equilibrium positions in the fcc lattice. The hardening by carbon in martensite is due to these distortions as the Fe atoms strongly react and lock dislocations.

It is the gliding of dislocations in crystal lattice structure of metals that produces ductility. The dislocations are induced by cold-working a material, and annealing removes dislocations.

 

[Astronuc]

I just received the November/December 2004 issue of Heat Treating Progress - the 2005 Thermal Source Book with Buyer's Guide. The magazine is published by ASM International, and is free to members.

The issue covers a lot about heat treating and particularly of steels. It seems to answer a lot of your questions.

I would encourage all material science/engineering students to become members of ASMInternational and other societies. Student memberships are quite inexpensive.

 

[PerennialII]

Here's some stuff about martensite transformation as well :

http://www.engr.ku.edu/~rhale/ae510/heattreat/sld019.htm
http://www.ce.berkeley.edu/~paulmont/CE60/heat%20treatment/sld013.htm
http://www.mete.metu.edu.tr/Facilities/Research/SMA/smatext/MART.htm

 

[el_hijoeputa]

I would encourage all material science/engineering students to become members of ASMInternational and other societies. Student memberships are quite inexpensive.

Your subscription is a student membership? I just want to know if the student membership get the printed magazine and other goodies.

 

[Astronuc]

I am a full (professional) member of ASM International.

When I was a student, I had a dual ASM/TMS student membership. For a very low fee, I received two (one from each society).

May I recommend that you check with the faculty of the Materials Science or Engineering Department. Hopefully they would have the applications.

If not, please visit the websites - ASM (http://www.asminternational.org/) and TMS (www.tms.org).

Your best bet may be to obtain membership through TMS -
http://www.tms.org/Students/Students.html -

Apparently, the joint student membership has been extended to the The American Ceramic Society (www.acers.org[/URL]). That's a great deal!