8 Reasons Steel Can Warp in Heat-Treat

Thanks Milo!

If you have knowledge/experience in similar changes during vacuum and/or inert or reducing atmosphere brazing, I'm ready to learn more. My most immediate interest is in brazing very thin stainless or super-alloys either alone or to thicker substrates.

Hmmm. Water contamination in quenching oil. That's one I wasn't aware of. Otherwise, I would think that you stress part design errors far too little in this discussion. A poorly designed part (large sectional changes or mass differentials, asymmetric features) will definitely impact the uniformity of heating, cooling and agitation, with some regions of the part being heated or cooled more rapidly than others, and uneven heat transfer with agitation. The first rule should be that the part must be DESIGNED with the treatment process in mind.

Very interesting information, thanks to Milo!

Discussion on quenching distortion brings to my mind challenges faced by Heat treaters of gears and bearing races which are especially prone to dimensional changes during hardening and quenching and can cause a number of problems during post-heat treatment manufacturing operations.

To minimize distortion the "quench press " concept is used and interested may refer to http://vacaero.com/News-Info-From-Industrial-Heating-Magazine/News-Info-From-Industrial-Heating-Magazine/Fundamentals-of-Press-Quenching.html to learn more.

Another very interesting process of quenching uses UCON quenchants, aqueous solution of non inflammable liquid organic polymer and corrosion inhibitor. The polymer is completely soluble in water and produces a clear homogeneous solution at room temperature. However at elevated temperatures separates from water as an insoluble phase. Upon cooling the polymer re-dissolves in to homogeneous aqueous solution. The product by Dow chemical is clearly described at

http://www.dow.com/PublishedLiterature/dh_0064/0901b803800641b1.pdf?filepath=ucon/pdfs/noreg/118-01567.pdf&fromPage=GetDoc

Thanks for this verry informative series of articles.

As a tool maker, my heat treating experience has been mostly with oil and air hardening tool steels, which are generally more forgiving than water quenched plain carbon steels. However, long thin sections of oil hardening steels (O1) are verry difficult to quench in oil without some warping, no matter how carefully I introduce them to the quench tank.

A trick that I learned years ago was to quench thin (up to 1/4 inch thick) parts of oil hardening steel between two thick plates of steel. The heat transfer rate is sufficient to achieve full hardness, and he weight of the top plate helps keep the part flat.

One caviat, you have to be fast. It is best to have a helper, one person to take the part out of the oven and place it on the bottom plate, and the other to immediatly place the top plate before the part can cool through the critical temperature.

Hi,

I would add a 9th cause: too rapid cooling.

Any cooling (and heating) -except if very very slow- will introduce stress by cool surfaces on warm interior and intermediate shear strength so yielding by shear a few mm below the surface. The the inner part cools then so the outer part will show tensile and the inner part compressive stress. This can be minimised by slow cooling (fast enough the get the desired hardness/strength).

There have been some measurements on general "stability", that is low speed (days, weeks, months, years) geometry change by creep from built in stress and residual instable phases. And mostly on the very early onset of deformation (micro-yield-stress): tolerable stress level for deformation of 10^-6.

Early work was by NASA: Stability of materials for spacecraft applications, Batelle, Columbus, ca.1965.

Then: Marshal and Maringer: Dimensional Instability, Pergamon 1973

Bernst: Werkstoffe des wissenschaftlichen Instrumentenbaus, DDR (eastern Germany) ca. 1975. (In German language only.)

(Both books out of print.)

Own bad experiences, stepwise improved: hardening of ball-bearing rings, coming out of the oven on a steel conveyor belt, laying flat and sprayed from above: heavy distortion,

same oven and each ring standing upright and sprayed from above: medium distortion,

same oven and rings separated more widely and sprayed from both sides: better but still better quality wanted,

same oven and rings sprayed from the inside with finer dispersed oil-showers below inert gas for fire-avoidance: optimum.

(Then optimum deep cooling and low temp annealing.)

Similar with chromium (13%) steels. Similar with materials for aerostatic and hydrostatic and ~dynamic bearings and high precision flexure hinges. All from hardenable steels and precipitation hardening materials.

RHABE

I would add to the list residual stresses from fabrication (welding & forming). Both of these processes result in large stresses lurking just below yield that will take the opportunity to relieve themselves when the heating process lowers the steel's yield point to below the residuals stress level. This can be uniform or non-uniform depending on the design and / or repair cycles.

I deal with welded and formed fabrications (pressure vessels mostly) rather than parts machined or stamped out of single blanks.

Solar Atmospheres is the preferred heat treatment service provider for my automotive parts manufacturing plant. Does any one else have any recommendations?