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Speaking of Precision

Speaking of Precision is a knowledge preservation and thought leadership blog covering the precision machining industry, its materials and services. With over 36 years of hands on experience in steelmaking, manufacturing, quality, and management, Miles Free (Milo) Director of Industry Research and Technology at PMPA helps answer "How?" "With what?" and occasionally "Really?"

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7 Causes of Quench Cracking in Steel

Posted August 03, 2010 8:56 AM by Milo

Failures of steel parts in service or production occur very infrequently. However, when steel parts fail, the consequences are dire.

Quench crack - this is not good!

Here are 7 ways that steel can fail as a result of Quench Cracking from heat treatment.

1. Overheating during the austenitizing portion of the heat treatment cycle can coarsen normally fine grained steels. Coarsely grained steels increase hardening depth and are more prone to quench cracking than fine grain steels. Avoid overheating and overly long dwell times while austenitizing.

2. Improper quenchant. Yes, water, brine, or caustic will get the steel "harder." If the steel is an oil hardening steel, the use of these overly aggressive quenchants will lead to cracking.

3. Improper selection of steel for the process.

4. Too much time between the quenching and the tempering of the heat treated parts. A common misconception is that quench cracks can occur only while the piece is being quenched. This is not true. If the work is not tempered right away, quench cracks can (and will) occur.

5. Improper design- Sharp changes of section, lack of radii, holes, sharp keyways, unbalanced sectional mass, and other stress risers.

6. Improper entry of the part/ delivery of the quenchant to the part. Differences in cooling rates can be created, for example, if parts are massed together in a basket resulting in the parts along the edges cooling faster than those in the mass in the center. Part geometry can also interfere with quenchant delivery and effectiveness, especially on induction lines.

7. Failure to take sufficient stock removal from the original part during machining. This can leave remnants of seams or other surface imperfections which can act as a nucleation site for a quench crack.

Finally, materials that are heat treated to very high strength levels, even though they did not quench crack, may contain localized concentrations of high residual stresses. If these stresses are acting in the same direction as the load applied in service, an instantaneous failure can occur. This will be virtually indistinguishable from a quench crack during an examination, due to its brittle failure mode, lack of decarburization on surface of the fracture, or other forensic evidence of a process failure.

When looking at quench cracking failures under the microscope, cracks and crack tributaries that follow the prior austenitic grain boundaries are a pretty good clue that grain coarsening and or its causes- overheating or too long time at temperature- occurred. Temper scale on the fracture surface helps the metallurgist know that the crack was present before tempering. Decarburization may show that the crack was open prior to quenching.

Thanks to WIP SAMI over at British Blades for the photo.

Editor's Note: CR4 would like to thank Milo for sharing this blog entry, which was originally posted here.

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#1

Re: 7 Causes of Quench Cracking in Steel

08/04/2010 12:59 AM

Milo -- This is great. Very practical information on steel metallurgy.

By the way, your PMPA bio gives some wonderful insights into what one of our "heros" is all about.

Ed Weldon

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#2
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Re: 7 Causes of Quench Cracking in Steel

08/04/2010 7:42 AM

Agree!

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#3
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Re: 7 Causes of Quench Cracking in Steel

08/04/2010 9:08 AM

Agreed Ed, Milo is all that and a pitcher of beer to boot. In fact I would be honored to buy him a pitcher if I ever got the chance, but only if I got to pick his brain while he drank it...=b.

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#4

Re: 7 Causes of Quench Cracking in Steel

08/04/2010 9:41 AM

Can I ask a question about pre-machining steel before hardening here ?

I'm making some drive shafts for my race buggy, they are 700mm long with Porsche/BMW CV's on either end, the drive shaft material is EN36A. I have access to a hobbing machine that I will be using to cut the spline in either end of the shaft/

What should I concentrate on when machining down the drive shaft? (between the CV splines)

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#5

Re: 7 Causes of Quench Cracking in Steel

08/04/2010 3:14 PM

There is another one: 8.- The steel to be quenched with internal defects: as porosity higher than standard, segregation, eutectic zones... you can have the seven causes OK, but if the steel has internal problems, you can have also quench craking....

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#6
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Re: 7 Causes of Quench Cracking in Steel

08/04/2010 3:26 PM

Of course the solution to that is don't by junk iron and call it steel. Milo did mention surface imperfections as possible nucleation sites, one could broaden that to ANY defect and have all of it covered.

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#7

Re: 7 Causes of Quench Cracking in Steel

08/04/2010 4:58 PM

Great compilation, Milo!

What I am interested in is:

there are some steels that tend to distort on hardening other ones have shrinkage but nearly no distortion. Is this known by experience only or is there a method to know from composition?

And: optimum cooling - to get sufficient hardness combined with least residual stress - is requiring the lowest possible cooling rate - to be derived from the TTT-diagram.

Is there a possibility to do a test or measurement during the cooling procedure to get an information about sufficient-insufficient cooling-rate?

RHABE

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Re: 7 Causes of Quench Cracking in Steel

08/04/2010 5:04 PM

RHABE, a metallurgist from a heat treating company here in town once told me that if you don't get at least 80% of UTS for Yield, you did not get a good quench. So after the fact a simple tensile test will tell you. Before the fact you are pretty much at the mercy of luck and your heat treater.

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#9

Re: 7 Causes of Quench Cracking in Steel

08/05/2010 4:31 AM

Curious to know how Caustic increases the strenght or the steel? Does this not have corrosion issues later when in sevice?

would appreciate any inputs in this regard.

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#10
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Re: 7 Causes of Quench Cracking in Steel

08/05/2010 11:21 AM

It works by both reducing the surface energy of the quenchant and more importantly removes scale and decarb from the surface of the workpiece that may act as an insulator. if the piece is cleaned properly afterwards, there is no corrosion impact.

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#11
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Re: 7 Causes of Quench Cracking in Steel

08/05/2010 12:36 PM

Rorschach,

thanks for the 80%UTS info.!

Concerning the higher hardness with salt or soda or else in solution I assume that better heat-conductivity and lower boiling activity (fewer insulating bubbles) is important too.

Spray-cooling is used too, is giving more uniform results but I don't know if yielding higher hardness. Should do so as boiling is eliminated and heat transfer perfected.

I saw people quenching very delicate parts in tin-?-alloys: no boiling, superfast cooling to around 150°C and then slowly cooling to achieve minimum internal stress.

RHABE

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#12
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Re: 7 Causes of Quench Cracking in Steel

08/05/2010 12:48 PM

Actually it was lead (they don't allow that any more generally in the US. the vapors make you stupid(er)...=b).

lead doesn't boil, has excellent conductivity and does not "wet" to the steel.

And yes, you are right liquid salt or Caustic Soda doesn't boil either which improves the conductivity to the quenchant, but since they have to be at elevated temps to be liquid (unless mixed with water or something else). the Delta T works against you I would expect.

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#13
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Re: 7 Causes of Quench Cracking in Steel

08/05/2010 4:15 PM

Hi R.,

the people who did this quenching in a tin-alloy told me that they never used lead as being too poisonous and too high in melting temperature.

They didn't want to tell me what was in the mixture besides the tin. There is a long list about low melting point alloys but all contain toxic elements.

I would like to try Ga-In-Sn but Ga is too expensive.

RHABE

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#14
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Re: 7 Causes of Quench Cracking in Steel

08/05/2010 4:23 PM

Possible candidates...

http://en.wikipedia.org/wiki/Low_melting_alloys

I'm betting on

Bi 58 , Sn 42

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#16
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Re: 7 Causes of Quench Cracking in Steel

08/06/2010 5:52 PM

Rorschach, Thanks for fielding those great questions while I'm down here trying to help my dad to find assisted living arrangements and get his arrangements straightened out..

I'll try to get back to every one but right now, I'm a little "occupied."

In the mean time, thanks for the great comments and questions all!

The lead bath/ Tin BAth may also be researched under "patenting"

The patenting of steel wires is a heat-treating process which consists of an austenitizinging phase and an isothermal quenching (holding) phase. Austenitizing is carried out at a temperature of about 900° C. and isothermal quenching at a temperature of about 500° C. Generally in a molten lead bath.

Milo

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#15

Re: 7 Causes of Quench Cracking in Steel

08/06/2010 12:00 AM

To what things are these best practices applied? Looked at page 6 and page 1099, but was wondering about now.

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#17

Re: 7 Causes of Quench Cracking in Steel

08/15/2010 11:21 AM

Dear Milo,

Thanks for information. There are many different alloys and different heat treatments required for them. Do you have any compilation of alloy wise heat treatments and the effects of same.

Is there any reference book available showing above data with the applications?

please oblige with information

kiran nawathe

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#18
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Re: 7 Causes of Quench Cracking in Steel

08/15/2010 12:48 PM

The ASM Desk Reference is a good reference book, as is "The Key to Steel". The "Worldwide guide to equivalent irons and steels" is one I've been meaning to pick up myself.

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#19
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Re: 7 Causes of Quench Cracking in Steel

08/16/2010 10:21 AM

Rorschach has given you the names of a couple of key references.I use them constantly.

If you are doing Heat treatment commercially, the Volume 4 Heat treating in the ASM Handbook series is a solid compendium on the subject. Link

For heat treat by alloy I consult the Isothermal Transformation of Austenite published by US Steel as well as Supplement to Atlas of isothermal transformation diagrams.

http://www.amazon.com/Atlas-of-Isothermal-Transformation-Diagrams/dp/B000BRQE98

In addition, I have some out of print materials and data that I developed or collected over my career that contain lots of the "magic."

Hanser Gardner published a book Mechanical properties of work materials by Dr. Edmund Isakov. It provides the regression equations for hardness vs tensile strength for a wide range of materials, I just use it asa check on the value of an mechanical property by grade when I use it.

http://www.amazon.com/Mechanical-Properties-Materials-Edmund-Isakov/dp/1569902941

Milo

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#20
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Re: 7 Causes of Quench Cracking in Steel

08/16/2010 12:50 PM

Dear Milo,

Thanks a galaxy. I am obliged.

kiran nawathe

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