4340 & ASM heat treaters guide

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Posted · Report post

I have a job to do where the specs call for quench and tempering to 241/277BHN. My machinerys handbook says to temper to 1300 for 241BHN which is awfully close to the quenching temperature. I am thinking that I will probably get inside that hardness range with just normalizing. I was told that was the way a similar forging was heat treated at the now defunct blacksmith shop at a local steel mill. These are just chisel points that are going to get welded on the end of pipes.
Are there charts in the ASM heat treaters guide with this sort of information? Are the charts more exhaustive than in the machinerys handbook? I have been thinking of buying this book but it is a little pricy if it does not have the information I want.

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Posted (edited) · Report post

1971 ASM Heat Treating, Cleaning and Finishing

4340 austenitising temp.-1500-1550F

BHN-standard ball 285= 29.9 rockwell C-scale 150-kg load brale penetrator,

temper 2 hours at 1200 degrees F, from austenitising temp. of 1500F quenched in oil, for section size 1/2"

This book gives hardness scale conversion charts but just happened to find a chart with your steel selection.

They say normalize@ 1600F for a BHN of 388 1/2" bar dia.


From Bethlehem Steel ,temper 1100-1225F

Hope this is helpful
Mark

Edited by markb

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Posted (edited) · Report post

If you just normalize, while it may be hard/soft as stated, it won't be martensite, and will not meet the spec's.

Edited by steve sells

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Posted · Report post

Dang Steve, beat me to it again! There are a number of thread up that quote BHN and Rc like thats all there is. That's only one measure. Sure, you want your cake to be soft and spongy, but it can still taste like xxxx.

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Posted · Report post

Thanks you guys I guess I should re read the metallurgy article here. I will quench and temper it, or send it out for heat treatment. I thought I could maybe save a step or a few dollars, seeing as this one was so soft. Some of the jobs I have been doing for one the steel mills specify quench and tempering. I have been heat treating them as the drawing specifies and the retired blacksmith from the steel mills own shop tells me just to normalize as them thats all they did.
This job is for the other mill.

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Posted (edited) · Report post

Maybe one of the metallurgists can help us a little here. I "believe" that normalized is a good state for hardening from but might not be a good "use" condition. A lot of normalized bar that I bought (in alloys) was N&T (normalized and tempered). I think we need more information. I'd probably do that if it was me making 'em. On the other hand, I have five fingers, no, I mean on the other hand it probably wouldn't hurt to send it normalized either, they are going to be welding on it right? They may be heat treating it after welding. I can't imagine them using it that soft. I've made chisels for almost every automotive foundry in the country at one time or another and they always wanted them HARD.

Edited by nakedanvil

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Posted · Report post

These are getting welded onto about 4' of pipe. But the assembly drawing just mentions preheat welding and post heat. They are for scraping or chipping at the coke ovens. These are 1 1/2" x 3" in cross section at the heavy end with a 5" taper to a sharp edge. I have made other coke oven scrapers with a 12" handle and 1/2 thick W1 and others with 1"x2" on a 4' handle with a hook on the other end. The others were tempered much harder. The ones I did the welding on I did the heat treatment after welding, these ones my customer is doing the welding.

I have been told the other ones I did lose there temper very quickly anyways. There is a reason for the 12' handle.

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Posted · Report post

I need to get a new thermocouple for my Gas Forge and I priced getting them heat treated at a different heat treat shop. At the $1.25/lb I just got them to do the heat treat, worked out to about $55.

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Posted · Report post

You will need to Q&T them to get martensite; that is what your specs said, I believe. 4340 is a Ni-Cr-Mo steel with deep hardenability. It is almost a tool steel. The moly in it will slow the tempering reaction down so if you cannot get the hardness you want at 1250F, temper for a longer time.

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Posted · Report post

Heat treat 1575 for 2 hrs per inch - then temper at about 1200F for 2 hours per inch - you might want to double temper too at a bit lower or higher temperature to get the hardness you want.

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Posted · Report post

Normalizing is often best viewed as an "air" quenched. It is typically performed by austenitizing, then cooling in air, either still air or forced air depending on the grade and desired hardness. 4340 is not going to form martensite during this process, but it does have the advantage of refining grain size and producing a fairly uniform (or "normal") microstructure. After forging, prior to any heat treatment, it is common for grains to be large and for there to be a mix of phases/microstructures present in the forgings, especially if it has a variety of cross sections. Normalizing gets you to a uniform starting point for the next heat treating step, which typically is austeniting and quenching for the purpose of forming martensite or bainite. So to answer Grant's question, yes, a normalized condition is often the preferred condition if subsequent heat treatment is required. In the specific case John brings up, the pieces will be welded. Now, welding can be done successfully on this grade provided the proper pre and post heats are used. As longs as these temperatures don't exceed the tempering temperature, you can weld without significantly altertering the properties develped during heat treatment. In the case of these chippers, the service environment sounds like it is pretty hot so failure is likely going to be do to softening over time and then wear or abrasion of the blade. A quench and temper will probably give a longer service life than a norm and temper, but there may be other factors influencing this that we don't know about.

John- If you are doing a lot of tool making, a copy of the ASM heat treaters guide would be a good investment since it does give a lot of charts and graphs showning hardness vs tempering temperature for hundreds of grades, including 4340, 4140 and the other tooling grades.

Patrick

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Posted · Report post

Thanks Patrick I think I will pick up a copy as soon as cashflow gets a little better. I have been shipping out quite a bit of my heat treating lately, and for a lot of my larger orders I think I may continue doing that. They have far better temperature controls than I do and as long as the order is big enough to be over their minimum I think is is more cost effective.

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Posted · Report post

John,

Look online for a used copy. You'll probably save yourself at least $100.

Patrick

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Posted · Report post

I picked up some 4340 at the metal recycle, I thought it would make good chisels. I don't think I am gonna be doin' any 2hr tempering.
Might there be a simpler process for a simple guy?

Thanx

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Posted · Report post

2Dogs,
If you are tempering to below 500F, 2 hours is like cooking a turkey (literally) Put it in the kitchen or toaster oven for the said time at the said temperature and you are good. This is common practice for knife makers and for making woodworking chisels.

So what kind of chisels are you making? The chisels mentioned in this thread are rather special in that they scrap out HOT ovens. This is different than a cold or wood chisel which you will want rather hard, but related to a hot chisel, only more severe of service.

Phil

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Posted · Report post

I have about 60' of 1" 4340 in 32" increments. I was hoping to make chisels for stamping hot metal, eye punches, maybe a few foot stamps.
Also for hammers what would the heat tx be? I would like to put this steel to use in its proper use range.
The research tells me "landing gear" is its primary purpose. I plan on making tools with it.

Sorry about the slow response

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Posted · Report post

I've made hammers out of 4340 and tempered them by running the color and quenching in oil. I love that stuff. Wish I had a bunch of it. For chisels, take it to a straw to bronze color and quench. For hammers, quench just as the color purple creeps up to the face.

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Posted · Report post

For hot chisels and punches used with a hand hammer, normalize and use. A full heat treat will get better life if they are used heavily, but the temper will continue to draw in service.

Phil

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