Steel selection starts with the application, not the datasheet. For a high-volume ABS injection mold, P20 (1.2311) at 30–34 HRC is usually the workhorse, but once you move into glass-filled nylon or PC with aggressive gate wear, you should be looking at H13 (1.2344) or a nitrogen-alloyed grade like 1.2083 for corrosion resistance. The common mistake I see is choosing steel by hardness alone. A 50 HRC S7 chisel may sound impressive, but its toughness profile is wrong for a large core insert that needs thermal conductivity and polishability. Match first to wear mode — abrasive, adhesive, or corrosive — then to hardness, then to machinability. For large molds, pre-hardened 718H saves you a heat treatment cycle and the distortion risk that comes with it, which matters when your cavity block is over 500 mm and flatness tolerances are tight.
Heat treatment is where good steel goes bad. For H13, austenitize at 1020–1050 °C, quench with a controlled interrupted oil or high-pressure gas quench, and temper at least twice at 550–600 °C to reach 48–52 HRC with retained austenite below 5%. Skip the second temper and you will see dimensional drift after EDM or nitriding. Speaking of nitriding, a 0.1–0.15 mm compound zone on a P20 cavity is fine for wear, but on H13 with sharp gate details, the white layer can spall — better to gas-nitride at 520 °C for a diffusion zone without the brittle compound layer. Always stress-relieve after rough machining: 550 °C for 2 hours, furnace cool, before semi-finishing. That single step prevents more cracked cores than any steel upgrade.
In practice, keep a heat treatment log tied to each mold plate — steel grade, lot number, austenitizing temperature, temper cycles, and measured hardness at three points. When a core fails at 20,000 shots, that log tells you whether it was a steel defect or a tempering miss. And don't overlook post-treatment handling: EDM recast layers on H13 need polishing or a 500 °C stress relief before service, or you are inviting heat-check cracks. For sourcing verified mold steels and heat treatment partners, visit MoldSo (www.moldso.com) to compare suppliers and specifications.