"Caveat Emptor" Is A Latin Saying And Means 'Let The Buyer Beware'

Posted by on

Our company is using a commercial heat treater and we’re getting good results. Why should we buy a Cress furnace and do the heat treating in house?

This is one question that we hear over and over again and there are arguments to steer the answer in both directions.  We’re going to look at several scenarios so that you can select the correct solution for your company.  Not all heat treaters operate this way, but with the high cost of energy, some are forced to take steps to keep costs low to stay competitive.

Background
You’ve created a tool, a part or series of parts in your machine shop that you need for your operations, or your customer’s requirements.   The designer has chosen A2 tool steel and specifies a hardness of 60 to 62 Rc is needed.  The part, or parts, could weigh 2 pounds, or 50 pounds and may have taken five hours , five days, or five weeks to make.  Regardless of the time, your company has invested time and money into producing the parts to satisfy a need.  You package the parts up, ship them off to the heat treater and ask them to be heat treated to 60 to 62 Rc.

Scenario Number One
The heat treater receives your package and analyzes what you’re asking for.  Their furnace can hold 500 to 1,000 pounds of steel per load.  So, your material goes into a queue waiting for other material from other customers in order to make the run profitable.

After two or three days, other materials do arrive, but there is only 120 pounds of A2 to run.  So, he loads the 120 pounds, which requires a 1750oF soak.  He has 75 pounds of D2 which requires an 1850°F soak, and 60 pounds of S7, which requires a 1750°F soak.

It’s not a full load, but it’s enough to process for this week and take care of their customers.  Of those steels the D2 is the most difficult to treat because of its high chemistry content; so the decision is made to process the load at the D2 temperature of 1850°.  It’s also decided that the size of the parts in the D2 load will be the controlling soak time factor.  That means your A2 and the S7 will get overcooked, creating excessive amounts of retained austenite and a very course grain structure will be created.  If your parts are somewhat larger in size, it will possibly offset a portion of the higher temperature, but the 1850°F is still going to have a very bad effect.

So, after the heat treated parts are quenched, the commercial heat treater will take your A2 and the S7 parts and put them into a mechanical freezer and freeze them at -150°F for a couple hours.  (Nearly 100% of commercial heat treaters have a mechanical freezer in a back room. It is also used to stabilize Aluminum parts.) This sub-zero treatment will convert some of the retained austenite into martensite and also make a partial correction to the hardness.  They will then perform a hardness test to determine at what temperature to temper the parts in order to produce the hardness that you requested.

When you receive the parts back, you may verify the hardness to be within specification and think that everything is fine.  Unfortunately, because of their action, the internal grain structure of your A2 and the S7 parts will still be large, coarse grained, and still contain a good deal of retained austenite that will refuse to transform.  The parts will go into service, but will never wear as they were intended to wear and you’ll soon be making the parts over again or, worst case, you may be looking for a new customer.

Bottom line of this scenario….. Hardness in ferrous metal is not what generates great wear resistance. Wear is the result of the finest-grained structure produced, martensite transformation, and excellent carbide creation and distribution.  If you want good results, use a Cress furnace and make sure your parts are processed correctly.

 DISCLAIMER OF LIABILITY
The material presented in this article is intended for general educational information only.  It should not be used for a specific application without careful analysis and study of the in¬tended use.  Anyone using this information or relying on it assumes all risk and any liability arising from their applications and use.

← Older Post Newer Post →