Heat Treating: Example of Error

[Joe in Oz]

Ray, thanks for sharing the fail and the thought process around what might have happened. I have only worked in Brass and Aluminum so Heat Treating is a mystery to me that someday I will need to figure out.

Since you mentioned brass, is there a way to harden brass, bronze or even copper? I know they will all work harden and can be annealed. But I'm looking for a way to harden them without distorting them to do so.

Cheers, Joe

 

[Ray C]

Since you mentioned brass, is there a way to harden brass, bronze or even copper? I know they will all work harden and can be annealed. But I'm looking for a way to harden them without distorting them to do so.

Cheers, Joe

Basically, yes... Virtually all metals can be manipulated to impact hardness to some extent. At a macroscopic level, the process is similar. Heat, Cool, Temper. In general terms, it's called precipitation hardening. The word "precipitation" means that the various elements of the metal are clumping (precipitating) into specific grain structures due to temperature manipulation. A similar process executed on two different materials might have opposite effects.

All of my books and study leaned toward ferrous materials because that's what I work with the most. I have complete professional reference data for ferrous metal and many data sheets for various types of stainless steel but none for copper alloys.

As for distortion, all professional guides for heat treating processes provides distortion information. I have a fairly good feel for ferrous materials but don't know the distortion characteristics of copper-based metal.

Here's decent article about copper alloy heat treating. http://www.totalmateria.com/Article71.htm

Ray

 

[RJSakowski]

When red hot iron is plunged into water, a blanket of steam forms on the surface doing a fairly good job of insulating. That plus the inner residual heat will slow the quench down to the point where full hardness won't be attained. Blacksmithing books all recommend swirling the work vigorously to dislodge the steam bubbles and properly quench the work.

 

[Ray C]

When red hot iron is plunged into water, a blanket of steam forms on the surface doing a fairly good job of insulating. That plus the inner residual heat will slow the quench down to the point where full hardness won't be attained. Blacksmithing books all recommend swirling the work vigorously to dislodge the steam bubbles and properly quench the work.

This is true. In ferrous metals, the effects from that phenomenon are likely to prevent a metal from reaching it's highest possible hardness which occurs right after the quench. All metal composition and treatment guides will list the metal's highest possible hardness. For example, the AISI standard states that 4140 has maximum hardness of 54 to 59 depending on the exact carbon content. Tools steels, which have tighter tolerances on component make-up, have a more precise number for max hardness. O1 for example is 64-65.

The effect from steam build-up around the part might lower the top-end number by a point or two. More likely, it will fracture the part. Some areas of the part will be in-contact with water (which is rapidly taking-away the heat) and some areas are surrounded by super-heated steam. The sharp contrast causes fractures.

A much bigger problem happens when the quench bath is way too warm to start with. If the bath started-out at 300F it will be much warmer after the quench and may not allow a rapid transition thru Martensite. If that happens, the part will be way more than just a few points way from the target.


Ray

 

[jwmelvin]

Is that—the Leidenfrost effect—why many (most?) quench procedures use oil?

 

[Ray C]

Is that—the Leidenfrost effect—why many (most?) quench procedures use oil?

Yes, it's the Leidenfrost effect. And many quench procedures use oil. Strong saline solutions will mitigate the problem as well.

FWIW, there are many kinds of quenching and there's a lot of science behind it. Most folks think of dunking in oil or water/saline solutions but, air blast and air/water mist are very common too. Then comes Austempering and Marquenching where quenching and /or tempering is done in several steps possibly ranging from molten salt to oil then oil or water. I've never concerned myself with those more complicated methods but, I do find it interesting. For very precise hardening needs, a metallurgist or mechanical/process engineer may tailor a procedure given the part's physical characteristics and material type. The whole purpose is to control the times the material spends at specific temperatures to control the microstructure transformations (hence the term TTT, time-temperature-transformation graph).

There are many phases but, for most ferrous materials, people are concerned with Austentite, Pearlite and Martensite. Each of those are products that form with a specific grain structure within the metal. By manipulating the phases at different speeds the final grain structure can be engineered for a specific outcome.

Ray