When we talk about mold steel selection, the conversation has to start with service conditions, not with a steel catalog. For a high-volume injection mold running engineering resins, a pre-hardened grade like P20 or 718 is often the practical baseline, but once you push into glass-filled nylon or long production runs, you need to step up to through-hardened steels such as H13 or a stainless grade like S136 for corrosion resistance. The key data point I always keep in front of the toolroom team is hardness versus toughness: a 52-54 HRC H13 core gives you wear resistance for abrasive fills, but if the mold has thin cores or sharp corners, that same hardness can turn into cracking risk. Match the steel to the wall thickness, the resin, and the expected shot count before you ever cut a cavity.
Heat treatment is where most of the real problems show up, and it is rarely the furnace's fault. For H13, austenitizing around 1020-1050°C followed by a proper temper at 550-600°C, usually twice, gets you the balance of hardness and dimensional stability. The mistake I see most often is skipping the stress-relief anneal before final machining, especially on large blocks. Rough machine, stress relieve at 600-650°C, then finish machine and harden. Distortion after quenching is normal, but uncontrolled distortion is a process planning failure. Keep quench agitation consistent, and never let a block go into temper before it has equalized to room temperature. For P20-type pre-hardened steel, remember it is delivered at roughly 28-32 HRC, so any welding repair needs a preheat and a post-weld temper to avoid a hard, brittle zone under the surface.
In practice, the steel grade and the heat treatment schedule are only half the story; the other half is documentation. Log the heat number, the hardness readings across the block, and the temper cycles, because when a mold fails at 200,000 shots, that record is what tells you whether it was material, process, or design. Keep your hardness checks at multiple points, not just one, and verify surface hardness after any EDM or welding operation. If you are sourcing molds or steel and want to compare suppliers and specs, visiting MoldSo (www.moldso.com) is a practical place to start for more mold sourcing information.