Tempering Soak, Temper Embrittlement, and Secondary Hardening

ScopeThe temperature comes from the steel's own tempering curve and is entered rather than derived, because the curve is a property of the specific grade: the same temperature produces quite different hardnesses in different steels, and no general relationship replaces the grade's own data. Soak time follows the section, with a stated minimum that governs on thin parts. TEMPER EMBRITTLEMENT IS THE FAILURE A HARDNESS TEST CANNOT FIND. Some alloy steels are susceptible in a range around the middle of the tempering scale, particularly on SLOW COOLING through it, and a part tempered there and furnace-cooled can come out at precisely the specified hardness with substantially reduced impact toughness. Nothing in the hardness reading says so. Where the grade is susceptible the specification requires cooling rapidly through the range, and that is a process requirement rather than a refinement to be dropped when the furnace is busy. SECONDARY HARDENING RUNS THE OTHER WAY AND CATCHES PEOPLE WHO REASON FROM STRUCTURAL STEEL. A tool steel such as H13 climbs back to a secondary peak in the middle of its range (about 54 HRC near 900 degF against 53 at 700) instead of softening steadily, because alloy carbides precipitate there, so the intuition that a hotter temper always means a softer part gets the direction wrong over part of the range. They are also commonly double or triple tempered, because each temper transforms some retained austenite which the next one then tempers -- and a single temper on such a steel leaves untempered martensite in the finished part, which is brittle and dimensionally unstable. Both of those are reasons the curve is read rather than reasoned from. This computes a soak time and screens an entered temperature against an entered susceptible range. It does not supply a tempering curve, an embrittlement range, or a soak rule for any grade, predict hardness from temperature, address the austenitising and quenching that precede tempering, model retained austenite or specify a multiple-temper cycle, evaluate toughness, or address stress relief, which is a different operation at different temperatures. The steel supplier's tempering data, the applicable process standard, and the metallurgist govern.

Formula or tablesoak time = section thickness x an entered rate against a stated minimum, with the tempering TEMPERATURE read from the steel's own curve and entered; the temperature is screened against an entered temper embrittlement range.

Source and editionTempering practice with the temperature taken from the grade's own tempering curve, which is entered because it is a property of the specific steel and no general relationship replaces it.

Free accessOne soak rule and one range comparison.

What governsEstimate. AHJ and licensed professional govern.

API field namestarget_hardness_hrc, curve_temp_f, section_thickness_in, soak_rate_hr_per_in, minimum_soak_hr, embrittlement_low_f, embrittlement_high_f, secondary_hardening, soak_time_hr, computed_soak_hr, embrittlement_span_f

• The tempering curve is entered it is a property of the specific gradethe steel supplier's data
• Embrittlement susceptibility is grade-specific and the range is enteredthe applicable process standard
• Hardness is the only thing confirmed toughness and retained austenite are nota metallurgical evaluation