Low and High Tempering of Steel

Your vocabulary is atypical and likely causing confusion.

When you say high tempering do you mean high temperature temper? or do you mean high hardness?

As one raises the tempering temperature after quenching to martensite, one relieves more of the stresses in the structure, allowing the material to relax, and thus lowering hardness and tensile and yield.

As one uses a very low temper temperature, one barely relaxes the striucture, leaving thematerial at very high hardness ,tensile and yield strength.

So as long as high and low refer to temperature of tempering, after the quench, the higher temperature results in lower mechanical properties; the lower temper temperature results in retention of higher mechanical properties.

Failure to temper results in brittle untempered martensite and usually stress cracking and part failure

milo

Hi John Thanks for the time taken to reply and share your experience. i am new to these and your answer helps in my understanding now. By the way, are you in the steel industry? and where do you work from? I am in singapore and am wondering if it is possible to connect with the steel profession people to network and make friends. Do drop me your contact, if you do not mind. Best Regards Alan

So what is the application? Climate may have a factor in choice. How tough to you need? Do you have access to a testing facility? Do you need a training film/ or Power Point Presentation? Form follows function.

Hi, Thanks for the time taken to reply and share your experience. i am new to these and your answer helps in my understanding now. The application is D2 steel material and on plastic mold steel such as SUS420J2. for the D2 steel material after reading the replies from other feedback, i can understand better now. As for the SUS420J2 material, if the plastic mold needs to be polish to mirror finish, I understand that tempering in low or high temper affects the polish results. How does this occurs? and why? would you be able to share your opinion? Do you have a training film or power point you can share? that would be greatly appreciated. By the way, are you in the steel industry? and where do you work from? I am in singapore and am wondering if it is possible to connect with the steel profession people to network and make friends. Do drop me your contact, if you do not mind. Best Regards Alan

Add to this there is a banned band for quite a few - if not all- at which temper embrittlement occurs.

Hi Tony, Thanks for the time taken to reply and share your experience. i am new to these and your answer helps in my understanding now. By the way, are you in the steel industry? and where do you work from? I am in singapore and am wondering if it is possible to connect with the steel profession people to network and make friends. Do drop me your contact, if you do not mind. Best Regards Alan

Allan

I am a telephone technician by trade. I just have a mechanical, building, inventing kind of interest. Most of my knowledge comes from doing metalwork at school, a few years hanging out with a retired fitter/machinist friend & some TAFE training in machining subjects.

I really do not have any formal training or expertise in heat treating of metals, just a broad basic understanding (pretty much all I have told you already!)

I am sure you will find plenty of others here who can help you with information & training materials.

Cheers

Tony

Hi Tony, Thanks for the reply and sharing your knowledge. I am in sales of Tool Steel and am working for a Japanese company. My boss are japanese and they are not good in explaining things in english so i turn to the internet to find answer to my questions. Once again, Thanks Regards Alan

Hi, Alan Lim!

First of all, allow me to welcome you to CR4. I hope you continue to find it fun and worthwhile to join in by reading and writing into all kinds of areas of engineering problems and ideas.

Here is a link where double tempering and the order of high and low or low and high is discussed.

http://www.eng-tips.com/viewthread.cfm?qid=233317&page=5

Mark

Hi mark, thanks for the reply and welcome note. By the way, are you in the steel industry? and where do you work from? I am in singapore and am wondering if it is possible to connect with the steel profession people to network and make friends. Do drop me your contact, if you do not mind. Best Regards Alan

Welcome to the forum

You must have some idea about the HT operation (ie Hardening and Tempering operation) by now.

To briefly re-cap, what has been already told above,

In hardening, steel, with sufficient amount of c (else no hardening ) is cooled from the upper critical temperature at a speed greater than what is called the critical cooling rate. When the steel is cooled at this rate, the carbon, in solid solution form in the hot metal, does not get the chance to get out and remains in this form in the cold metal too (and this % is beyond the solubility of c in fe), While doing this, it passes first through the Austeniete phase and then at room temperature in the needle shaped martensite phase.

This martensite phase is very hard and due to its shape is very brittle. It also has some Austenite in it, and is measured as the retained Austenite (must have come through this term) it is the percentage of Austenite which decompose into martensite.

Theis high brittleness, though is wanted for some application (such as wear resistance, etc), but makes the material very brittle and not fit for stressful life .

To improve the toughness (the fatigue properties as well as some mechanical properties) , the tempering is done. In this the hardness is reduced but the mechanical properties are dramatically improved.

In tempering operation, the component is heated to a temperature much lower than the critical temperature and then slowly cooled to another temperature and then quenched in oil/ quench media.

In tempering, what we land up is called the tempered martensites. The martensites and austenites start diffusing the excess carbons from it and these carbons come out in forms of Fe₃C and form pearlites and cementites.

The tempering temperatures as well the effects tempering stage-1 to stage-4 may be looked into this article.

However the forbidden range of tempering, normally will be available in the specific material data-book.

http://steel.keytometals.com/Articles/Art127.htm - sorry, link no longer available

Please do not take the above post as gospel. There are several inconsistencies, errors, and confusions.

This is a partiucularly egregious example:

"In tempering operation, the component is heated to a temperature much lower than the critical temperature and then slowly cooled to another temperature and then quenched in oil/ quench media."

This is not true in description, process steps, nor the vagaries and omission of imputed facts ("cooled to another temperature") Huh? After soaking to uniformity at tempering temperature, the parts slowly cool to ambient temperature.

Tempering, involves the furnace treating of the as quenched parts uniformly to some temperature below the critical point, then allowing them to cool slowly in air. There is no oil quench after tempering!!! This is important!

The quote above does not describe either Austempering (isothermal transformation to bainite; really a quench); nor Martempering, which involves a salt bath and air cooling.

In oil tempering, AFTER AUSTENITIZING AND QUENCHING, small parts or wire are reheated in oil up to about 350 degrees to get uniform temperature through section in mimimum time to to prevent scale formation at temperature. NO post temper oil quench.

There is no need to oil quench after tempering. I have no idea where this came from, but in my 30 years of metallurgy of bar, wire, rod, and part testing and manufacture, I have never encountered "oil quenched after tempering."

The only oil quench used is after austenitizing to create the metastable martensite phase. As-Quenched martensite is very hard and brittle. If allowed to remain in that highly stressed condition, cracks will tend to form in all but the lowest carbon anlyses. Tempering immediately at a temperature that will produce the desired mechanical properties by relieving stresses and so will prevent those cracks from developing.

Respondents that have not done work or education in metallurgy should probably abstain from giving "recaps" in this field.

Someone taking incorrect advice on a key component or process could end up getting someone hurt or killed. In the first place, ddo no harm.

Engineering is serious pursuit. If you haven't done the classwork or the job, please don't post and possibly get someone hurt.

BAD ANSWER!

milo

I stand by my comments. Actually you did go into details of microstructure etc.

oil quenching after tempering I think not unless you meant but didn't say double quench and temper...

I agree that cryogenic treatment may be indicated to pull out retained austenite and retemper. I consulted on a bike lock job many years ago where the locks which had a guarantee were inexplicably failing. High warranty costs. A good shot with a co2 fire extinguisher could chill them down to the point where a sharp blow with a hammer would crack and voila lock would open. We adjusted the alloy and the heat treatment cycle.

oil quench at tempering temperatures- no thanks I'll pass. placebo at best. or proxy for homogenization at temperature in a process being cycled to quickly.

Noone said that you or "we" have a habit of thinking that you or"we" know everything.

But speaking for myself, I do know what I know. And in my field, I can recognize valid communications as well as those that are not congruent with first principles.

If your secondary quenching is after a second austenitizingtreatment following a first or prior Q&T, I get it, BUT THATS NOT HOW IT WAS TYPED. If the point is that an oil quench after tempering to a temperature is a normal tempering process, I disagree.

peace.

milo

Dear Milo,
Here we go by concept (for details there was a link already provided from K2s). Infact if you go by wording then as peryour first post
So as long as high and low refer to temperature of tempering, after the quench, the higher temperature results in lower mechanical properties; the lower temper temperature results in retention of higher mechanical properties.

is correct? Yes if you mean Strength is the only mechanical property. Infact if you want retention of higher mechanical properties vis vis hardening, the tempering is not necessary.

Again to quote the first article on google.

A high-rate cooling from a tempering temperature above 600°C, for instance, water-cooling, can prevent the appearance of temper embrittlement. On the other hand, a quick cooling on tempering at 450-600°C cannot prevent temper embrittlement. Thus, entering the dangerous temperature interval from either "below" (on heating and holding at that temperature) or from "above" (on slow cooling) can produce the same result.

Check agaist your comment

Tempering, involves the furnace treating of the as quenched parts uniformly to some temperature below the critical point, then allowing them to cool slowly in air.

Why would you want to quench martensite for a second time?

Tempering, involves the furnace treating of the as quenched parts uniformly to some temperature below the critical point, then allowing them to cool slowly in air.

This is exactly how it's done. One must be vigilant in that air currents within the shop can affect this cooling in such a way as to cause the casting to crack. With some large castings the slightest temperature differrance will cause micro fractures.

Hi Milo, thanks for the feedback and comments. i really appreciate your feedback regards alan

I fully agree: No "quenching" is done after tempering (just before).

The only question is the cooling media, mostly air, but some steel manufacturers give properties for air cooling and oil cooling after tempering but this cooling isn't at all a "quench"

I think to call "quench" this cooling from tempering temperature is wrong and confusing to those (as OP seems) not experts on heat treatments.

Kind regards

Hi all, I follow many a thread on CR4 but very rarely contribute (My fault, I apologize).

So far the hardening and tempering of steel has been covered very well from a technical point of view so I thought I'd throw in a very NON TECHNICAL perspective.

I have an old friend that makes a lot of hunting knives. His money supply is short so he uses spring steel instead of tool steel because it's cheaper. I'm not sure, but I think many a leaf spring has gone into his knives. For hardening and tempering he as a simple technique that has served him well. When he's done with his shaping he slowly heats up a knife blank until it no longer sticks to a magnet, then quenches it in ice water. Along side his water bucket he has a lead pot. He heats up the pot until the lead has just melted, dips the knife blank in for a second or two, pulls it out, and lets it cool down slowly in air. All I can say is his knives are tough and hold an edge well.

Maybe it's a question of my poor english language education (I was taught mainly in spanish), but as far as i know, the process to achieve a steel structure in which strength and toughness are enhanced is called "quenching and tempering". The process begins with heating the piece above the Ac3 temperature to get a full austenitic structure. Then the metal is more or less quick cooled (cooling speed depends on the CCT curves of the specific steel) to achieve a non equilibrium transformation, normally by sliding planes, and austenite becomes a non equilibrium structure called martensite much harder than ferrite and more brittle too.

After "quenching" to improve the material toughness, it's submitted to what is called "tempering" which consist in heating the steel below transformation point, just to get a more softer structure, by allowing the carbon supersaturated structure (martensite)release some of this excess carbon becoming "tempered martensite". As the tempering temperature is higher, the more tough is the material (generally speaking and not taking into account some brittleness which may occur at intermediate temperatures near 250ºC or 400ºC) but for a first approach it's valid.

Here is a graph to illustrate the effect of tempering temperature. As it's i spanish, for those who doesn't understand it well, the left "y" axis is tensile strength and th right one is notch toughness. The corresponding curves are "R" and "P" and abscissa axis is tempering temperature in ºC. Dotted lines P1 and P2 are examples of tempering brittleness.

The terms "high temper" or "low temper" IMHO are a bit confusing even I've read it in some technical articles and books related to "high temperature tempering" and "low temperature tempering" which I consider more technically adequate.

So, resuming the answer to your question is: Who knows! It depends on what specific steel you are dealing with and to which specific temperatures you will do the tempering.

Kind regards