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We had two large rolls, 18 inches (460 mm) in diameter by 42 inches (1,067 mm) long with a wall thickness of approximately 3.5 inches (89 mm) that were heat treated, and the heat treatment failed to produce the desired results. The parts are made from S90V powdered metal. The heat-treat furnace records do not indicate an issue, but the heat treatment resulted in a hardness of RC 52, which was deemed acceptable. After material removal of 0.08 inches (2 mm) radially, the roll hardness was measured to be 24-53 HRC, dependent upon location. Can you help explain what might have happened?
In reviewing the information you've provided, it appears that the parts were not properly heat treated. Crucible CPM S90V (2.3% C, 14.0% Cr, 9.0% V, 1.0% Mo) is a martensitic stainless steel in which there are a large number of vanadium carbides (as opposed to chromium carbides) for improved wear resistance. It is capable of being hardened to around 58 HRC.
This material should be austenitized at 2100-2150°F (1150-1175°C) and quenched after being held at temperature in the neighborhood of 120 minutes (given the thickness of your part). A variety of quenchants (salt, an interrupted oil quench, positive-pressure gas quenching at 4 bar or greater, or air cooling) are acceptable provided a minimum cooling rate of 150°F/minute (80°C/min) to below 1000°F (540°C) is achieved. The part should then be cooled to below 125°F (50°C) prior to tempering. A double (2 + 2) temper at 400-750°F (200-400°C) is recommended. The part should be held at temperature for a minimum of 2 hours for each temper. Tempering between 800-1000°F (425-540°C) is not recommended due to embrittlement concerns.
From the data you provided, the austenitizing temperature was 2075°F (1135°C), the soak was only for 30 minutes and a 2-bar gas quench was used. Double tempering at 550°F (290°C) and 575°F (300°C) was then done. The soak at austenitizing temperature appears to have been inadequate for all of the alloying elements to go into solution, and the quench was too slow to fully transform the material. To confirm this, a detailed metallurgical analysis should be performed.