Heat-Treat Soak Time and Furnace Load

How long the charge has to sit at temperature, and how much energy it takes to get there. Soak time is governed by SECTION THICKNESS, not by weight. A hundred pounds of half-inch bar and a hundred pounds of two-inch bar are entirely different soaks, because what has to happen is that the CENTER of the thickest section reaches temperature and then stays there long enough for the transformation to complete. The common rule -- roughly an hour per inch of section at temperature, after the part is through-heated -- is a convention with a lot of process-specific variation behind it, and it is separate from the time it takes to get the part hot in the first place, which is why both terms are printed. The thickness leverage is the whole lesson: a 4 in section doubles both the through-heat and the soak, for a charge that may weigh exactly the same. In heat treating, geometry beats weight every time. The energy side sizes the furnace, and the surprise is usually how much of the furnace's input never reaches the work: wall losses, opening losses, atmosphere, and fixturing all consume a large share, and 50% to 70% overall efficiency is common on a batch furnace. A 500 lb charge of alloy steel taken to 1,550 F from room temperature absorbs 88,800 BTU, and delivering it during a two-hour ramp at 60% efficiency needs about 74,000 BTU/hr, or 21.7 kW -- which is the number that says whether the shop's furnace and its circuit can run this charge on schedule. Time and energy only, and the soak rule is a convention rather than a metallurgical prediction. THIS DOES NOT SPECIFY A HEAT TREATMENT. The austenitizing temperature, the soak time, the quench medium, the temper, and the resulting properties come from the steel's own specification and the applicable process standard (AMS 2759, ASTM A991, or the customer's own), and they depend on the alloy, the prior condition, the required hardness and toughness, and the section size. It does not address atmosphere and decarburization, fixturing and distortion, ramp rates and thermal shock on thick or complex sections, pyrometry and thermocouple placement, or the survey and calibration requirements a certified shop works under. The steel's specification, the process standard, and the metallurgist govern.

through-heat = rate x section thickness; soak = rate x section thickness; total at temperature = the sum; charge heat = weight x specific heat x (soak - start); furnace input = charge heat / (through-heat hours x furnace efficiency).

The section-thickness soak convention used in heat-treating practice (through-heat and soak each proportional to the maximum section, commonly about one hour per inch at temperature), by name, with charge heat from the sensible-heat relation and the furnace's overall efficiency applied to the input. THIS SPECIFIES NO HEAT TREATMENT: temperature, soak, quench, temper, and properties come from the steel's specification and the applicable process standard. The metallurgist governs.

A thickness rule stated as a convention with its rate entered by the user, plus a sensible-heat calculation; no process specification or property table is reproduced.

Estimate. AHJ and licensed professional govern.

Field names used by the API: charge_weight_lb, section_thickness_in, soak_temp_f, start_temp_f, through_heat_rate_hr_per_in, soak_rate_hr_per_in, specific_heat, furnace_efficiency, through_heat_hr, soak_hr, total_at_temp_hr, charge_heat_btu, furnace_input_btu_hr, furnace_input_kw

• Thickness, not weight the center of the thickest section governs; doubling the section doubles both termsheat treating practice
• The hour-per-inch rule is a convention process-specific variation is large and the specification governsheat treating practice
• Most of the input is lost wall, opening, atmosphere, and fixturing losses put batch-furnace efficiency well under three quartersfurnace practice