440C Stainless and Carbide Distribution 1

[Schuyler]

Is there any heat treat hardening process (i.e., specific times, temperatures, etc.) for 440C stainless steel that will minimize the presence of carbides and make sure the carbides are finely dispersed within the matrix and not distributed along the grain boundaries.

 

[kenvlach]

Please see MIL-H-6875H HEAT TREATMENT OF STEEL, PROCESS FOR which is a free download from ASSIST:

http://assist2.daps.dla.mil/quicksearch/

In this spec., 440C is a Class B - Martensitic corrosion-resistant steel. The specific heat treatment procedure for 440C is given in TABLE IB. This should ensure a proper microstructure. Note that Paragraph 4.3.3.2 requires testing to verify freedom from intergranular attack.
Please also note that a neutral furnace atmosphere (neither oxidizing nor reducing) is required per paragraph 3.1.1.1.

 

[metrat]

It is fairly important to properly cool 440C to aviod carbide precip at the grain boundaries. There are a number of documents that give good guidelines, AMS-Mil-H-6875 or AMS-2759. Generally, if 440C is cooled too slowly from the hardening temp. through about 1100F, you will get precipitation. Typically, depending on section thickness, etc., a good gas quench is acceptable (nitrogen or argon), but an oil quench would also work. Process I've used is:

Harden at 1940F, soak based on section thickness, about 45 - 60 minutes.
Oil quench
Sub zero @ -100F (if part stability is important)
Temper at 350 F or other temperture based on final hardness requirement.

Hope this helps.

 

[Maui]

Schuyler, 440C is a martensitic stainless steel as kenvlach pointed out. This alloy was designed to possess a combination of good wear resistance and moderate corrosion resistance in mild environments. Precipitation of carbides along the grain boundaries may occur if the alloy is quenched too slowly from the austenitizing temperature during heat treatment. In order to minimize this effect, you will need to cool the entire cross section below 1000F in 4-5 minutes or less. The hardening practice that is usually recommended is as follows:

Preheat: 1100/1250F, equalize, 1400/1450F, equalize.

High Heat: 1850/1900F, hold 30/45 minutes at temperature.

Quench: Air, positive pressure vacuum, or interrupted oil. Cool to 150F prior to tempering.

Temper: 400-800F, hold 1 hour per inch of thickness, 2 hours minimum each temper. Temper at least twice. Note that tempering between 800 and 1100F should be avoided due to a decrease in both toughness and corrosion resistance. For this reason this temperature range is referred to as the sensitization range.

Cryogenic Treatments: Refrigeration treatments may improve long term dimensional stability by transforming retained austenite. Refrigeration treatments should generally be performed after the first temper, and must ALWAYS be followed by a temper.

Note that your aim hardness will determine what your your austenitizing temperature and tempering temperatures will be. I hope that this answers your question.


Maui