Tool Steel Heat Treatment Troubleshooting Chart
Seven defects that send a hardened tool back to the furnace or to the scrap bin, with the cause behind each one and the check or the change that corrects it. Hardness that comes out high or low, decarburization, uneven hardness, distortion, size change and cracking during hardening are all covered.
| Possible cause | What to check or change |
|---|---|
| Higher hardness than specified | |
| Poor furnace temperature control | Check and calibrate furnace controls regularly |
| Surface carburization | Anneal and/or harden in furnaces with controlled neutral atmospheres |
| Using hardening temperatures that are too high | Do not exceed the manufacturer’s recommended hardening temperature range |
| Overheating and/or oversoaking at the hardening temperature | Do not exceed the manufacturer’s recommended hardening soak times, which vary with different furnace types |
| Insufficient and/or short cycling of the tempering operation | Select the highest tempering temperature consistent with the required hardness. Soak at the tempering temperature for 120 min/in. (4.72 min/mm) of thickness at heat. Double and triple temper air-hardening tool steels |
| Lower hardness than specified | |
| Poor furnace temperature control | Check and calibrate furnace controls regularly |
| Surface decarburization | Anneal and/or harden in furnaces with controlled neutral atmospheres. |
| Using a hardening temperature that is too low | Stay within the manufacturer’s recommended hardening temperature range |
| Insufficient soaking at the hardening temperature | Preheat thoroughly before hardening. Follow recommended austenitizing (hardening) soak times |
| Quenching too slowly and/or quench interruptions | Quench uniformly, without interruption. Employ a uniform fan blast for air-hardening steels when section size requires more than a still-air quench for proper cooling |
| Tempering at too high a temperature range for the desired hardness level | Follow recommended tempering guidelines |
| Uneven hardness across the surface | |
| Surface decarburization and/or surface scale | Be sure all decarburization is removed prior to hardening. Anneal and/ or harden in controlled neutral atmospheres, or when these are not available, use stainless foil wrap. Seal all furnace openings to keep out air. Keep furnaces in good repair so they can be properly sealed |
| Nonuniform quenching | When air quenching large sections, use a uniform fan blast to accelerate cooling. Small section sizes may be cooled in still air in a place where air circulation is not impeded. Liquid quenchants should be agitated and kept cool possibly with a chiller. Keep liquid-quench baths clean. Use brine solutions instead of water quenchants |
| Decarburization and scale on the surface | |
| Failure to remove surface decarburization from barstock | Use decarburization-free steels whenever possible. Check and remove surface scale and decarburization as recommended from as-rolled stock |
| Failure to heat and soak in a neutral furnace atmosphere during hardening | Anneal and/or harden in a controlled neutral atmosphere, vacuum, or neutral salt furnace. |
| Distortion, bending, bowing or twisting | |
| Complicated design configurations | Design part with minimal stress raisers, avoiding sharp internal corner sections, thick sections adjacent to thin sections, blind holes, and holes with thin wall sections |
| Mechanical stresses from cold working (machining) were released | Stress relieve prior to hardening to eliminate mechanical stresses |
| Insufficient part fixturing and support in the furnace | Do not overload furnaces. Support the workpiece properly in the furnace, suspending it vertically whenever possible. Clamp long, thin workpieces to support plates whenever possible |
| Thermal shock from heating to the hardening temperature too rapidly | Preheat thoroughly before hardening. Heat to and soak uniformly at the hardening temperature |
| Failure to heat and soak uniformly at the hardening temperature | Preheat thoroughly. Do not overload furnaces. Soak thoroughly at the hardening temperature |
| Nonuniform quenching | Use air-hardening steels whenever possible. Quench uniformly |
| Size change, shrinkage or growth | |
| Incomplete and/or nonuniform transformation of austenite to martensite | Quench thoroughly and uniformly. Do not short cycle the tempering operation. Double and triple temper air-hardening steels. Employ subzero and cryogenic quenching to get complete transformation (austenite to martensite) |
| Cracking during hardening | |
| Inaccurate furnace controls allow overor under-austenitization | Check and calibrate furnace controls |
| Design configurations create stress raisers | Simplify part designs that incorporate stress raisers |
| Severe mechanical stress concentrations | Stress relieve parts that have been severely cold worked before heat treatment |
| Carburization and decarburization | Anneal and/or harden in furnaces with controlled neutral atmospheres |
| Grain coarsening from overheating and oversoaking make steel brittle | Preheat thoroughly before hardening. Do not oversoak or undersoak |
| Nonuniform quenching | Quench uniformly. Use air-hardening steels whenever possible |
| Short cycling (insufficient) tempering after quenching | Temper immediately after quenching. Avoid tempering at temperature ranges under 400° F (204° C). Double and triple temper air-hardening steels |
Source: Tool and Die Making Troubleshooter (R. M. Leed), chapter 8 troubleshooting list, from Bethlehem Steel heat treating practice.
Reading the table
Most of these defects trace back to one of two habits. A short cycle skips the soak and leaves the core below the hardening temperature, and loose furnace control lets the part sit above or below the target. Plan the whole cycle of preheat, austenitizing soak, quench and temper, then hold the schedule. Where the part has to be stress relieved before hardening, the cycles are in the annealing and normalizing chart.
Cracking is the defect with the least warning. Steels that transform slowly carry less stress through the quench, so an air-hardening grade is the safer choice for a part with sharp corners or mixed sections than an oil-hardening grade. The quench cracking page covers the mechanisms, and the list of D2 heat treatment mistakes covers the shop habits behind most of them.
A part that reads low at the surface usually lost carbon rather than hardness. Decarburization shows on a ground face as a soft skin a few thousandths deep, and a file or a Rockwell test on the surface will miss it once the skin is removed. Check the hardness on a freshly ground face before blaming the steel, and use the hardness testing methods when a file is not enough. Values read on Vickers or Brinell equipment convert in the hardness conversion calculator.
Size change is not a defect. Every tool steel grows a little in hardening and D2 moves least of all, so a die that has to hold a tolerance is given finishing allowance and ground after heat treatment. The expected growth for each family is in the size change chart, and the stock to take off a hot-rolled bar to reach clean metal before machining is in the machining allowance chart.
Section size decides how fast a part has to be quenched to harden through, and a quench that is faster than the section needs adds stress for nothing. The section each grade will harden through is in the hardenability chart. A furnace with a pyrometer and a chart still needs a cross-check on the part itself, which is what the heat color chart is for. Cracks that appear after the die is in service rather than after the quench often start at an EDM white layer or an unrelieved grind, so check the finishing steps as well as the furnace.
The cycle for one grade is in the D2 heat treatment guide, with matching pages for A2, O1, S7, H13 and M2.
Preheat in two steps
A part that goes straight into a furnace at the hardening temperature develops thermal shock before it develops hardness. Bring it through one or two preheat steps first, and hold long enough for the core to reach temperature, not just the surface.
Document the cycle and keep the certificate
A heat treat record that carries the austenitizing temperature, the soak, the quench medium and the tempering cycles is what settles an argument about a failed part. The heat treatment guide sets out the fields worth recording, and the mill certificate proves the chemistry that went into the furnace.
Separate the cause from the design
A part that cracks at the same corner every time is a design problem rather than a furnace problem. Stress raisers, sharp internal corners and thick sections next to thin ones fail in heat treatment before they fail in service, which is why grade selection starts with the geometry of the tool.
Check the quench medium
Oil that has been in the tank for years cools more slowly than the grade needs, and a still-air quench on a heavy section cools unevenly. Keep liquid quenchants agitated and cool, and use a fan blast on large air-hardening sections. A part that reads low in the core but high at the corner is usually a quench problem.
Related reference pages
Tool steel forging temperature guide · D2 austenitizing temperature · Tool steel hot hardness chart · Tool steels for cracking resistance · D2 tool steel machining guide · Why D2 chips and cracks
Sources: Tool and Die Making Troubleshooter (R. M. Leed, Hanser Gardner), chapter 8 troubleshooting list, itself drawn from Bethlehem Steel heat treating data and brochures. Reference data for comparison only. Confirm the cycle with your heat treater and the mill data sheet before production. Aobo Steel supplies tool steel in the annealed condition, and hardening remains the customer’s heat treatment.