440A Stainless Steel Heat Treatment

440A in the 440 stainless family

440A is the lower carbon grade of the 440 stainless family, carrying 0.70% of carbon against the 1.05% of 440C. Chromium at 17.00% is what holds the corrosion resistance, and the carbon figure is what sets the hardness the grade can reach. The grade is taken where the part needs more toughness and easier grinding than 440C gives, and a lower working hardness is accepted in return. The tempering line below falls from the first row, so the hardness is set by the quench and the temper is used to trade part of it back for toughness and stress relief.

The card is shop reference information rather than a specification. It prints one preheat step, one hardening temperature and two quench media, then a tempering table with a single hardness figure on each row and no tolerance band anywhere. A first run of parts from a new heat of steel is worth treating as a trial before the cycle goes onto production work.

Chemistry of 440A

Elements the reference card lists, percent

Source, heat treating reference information for 440A, printed page 201.

The card prints one figure for each element rather than a specification range. Vanadium and tungsten are listed on the card with no figure against them, so those rows are left out of the table instead of being printed as blanks. Phosphorus and sulfur are printed as maxima.

Chromium at 17.00% carries the corrosion resistance of the grade, and carbon at 0.70% is what the carbides are built from. Manganese at 1.00% and silicon at 1.00% are printed as single figures, molybdenum at 0.75% is the only carbide former beside chromium, and phosphorus at 0.04% and sulfur at 0.03% are printed as maxima. Vanadium and tungsten are listed on the card with no figure against them, so those rows are left out of the table. Composition data in the table is for general reference only, actual values vary by standard, mill and heat number, so confirm the figures against the material test certificate of the heat or ask Aobo Steel.

Hardening 440A

Preheat, hardening temperature and quench

Source, heat treating reference information for 440A, printed page 201.

The card prints one preheat step, one hardening temperature and two quench media for the grade, with no section-size condition against any of them, so the figures stand for the load as written.

The preheat of 1400 °F (760 °C) brings the load up before the austenitizing temperature, and the card prints no condition against it, so the step stands for every load. Hardening runs at 1900 °F (1040 °C) followed by an air or an oil quench. Air is the slower of the two media and the one that holds distortion down on thin sections, while oil is used where the section is heavy enough to need faster cooling. The card prints one hardening temperature, so a shop running a different austenitizing window is working outside what the card gives.

Tempering response of 440A

Hardness left by each tempering temperature

Source, heat treating reference information for 440A, printed page 201.

The card leaves the as quenched row without a figure, so that row is left out of the table. Tempering runs from 300 °F to 800 °F in 100 °F steps with one Rockwell C figure on each row, and the hardness falls from the first row onwards with no second rise.

The card leaves the as quenched row without a figure, so the first hardness the table gives is 57 HRC at 300 °F (150 °C), and the line falls from there to 50 HRC at 800 °F (425 °C). There is no second rise on the curve, which follows the analysis of the grade, since it carries no cobalt and only 0.75% of molybdenum. The temper is therefore not where this grade builds its hardness. Tempering between 300 °F and 500 °F holds the part between 54 and 57 HRC, and every step above 500 °F trades hardness away for toughness and stress relief. The card prints one figure on each row and says nothing about the number of tempering cycles, so treat the line as the response of the grade and settle the cycles with your heat treater before the parts go into service.

Related reference data

The grades either side of this one in the stainless family are on the 420 stainless steel heat treatment page and the 440C stainless steel heat treatment page, and the supply condition of the higher carbon grade is on the 440C stainless steel page. Hardness and property data for the family sit on the 440C stainless steel properties page and the martensitic stainless steel properties chart, and the choice against a cold work tool steel is worked through on the D2 and 440C comparison page. Tempering bands across every tool steel group are on the tool steel tempering chart.

Reference data compiled from Heat Treatment, Selection, and Application of Tool Steels (William E. Bryson, second edition, Hanser). The chemistry, the hardening cycle and the tempering response for 440A come from the heat treating reference information on printed page 201.