440B Stainless Steel Heat Treatment

440B in the 440 stainless family

440B sits between 440A and 440C in the stainless family, carrying 0.85% of carbon against the 0.70% of 440A and 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 a part needs more wear resistance than 440A gives and stops short of the hardness 440C reaches, and it is often the grade a shop moves to when plain 440A is losing its edge too quickly. The tempering line below sets the working hardness, so it is read before the cycle goes onto the job.

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 440B

Elements the reference card lists, percent

Source, heat treating reference information for 440B, printed page 202.

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 0.85% 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 440B

Preheat, hardening temperature and quench

Source, heat treating reference information for 440B, printed page 202.

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 1200 °F (650 °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 440B

Hardness left by each tempering temperature

Source, heat treating reference information for 440B, printed page 202.

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 line dips at 600 °F before it comes back up.

The card leaves the as quenched row without a figure, so the first hardness the table gives is 58 HRC at 300 °F (150 °C). The line falls to 53 HRC at 600 °F (315 °C) and then comes back to 54 HRC at 700 °F and 800 °F. That rise is one point on a single figure line, so the second half of the table reads as flat rather than as a second hardening peak, and the molybdenum content is not high enough for the temper to be where this grade builds its hardness. Tempering between 300 °F and 500 °F holds the part between 54 and 58 HRC, and every step above that 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 grade below this one in the stainless family is on the 440A stainless steel heat treatment page and the grade above it is on the 440C stainless steel heat treatment page, while the supply condition of the higher carbon grade is on the 440C stainless steel page. A hardness comparison across the family sits on the 440C stainless steel properties page and physical property data on the martensitic stainless steel properties chart, with chemistry for every grade on the stainless steel composition chart. The choice against a cold work tool steel is worked through on the D2 and 440C comparison page, and 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 440B come from the heat treating reference information on printed page 202.