Complete Knowledge of Imported Mold Steel for CNC Machining

• 1
  What imported mold steel really means
• 2
  The four families you will actually machine
• 3
  How imported mold steel behaves in the cut
• 4
  When to specify imported mold steel, and when not to
• 5
  Heat treatment and the distortion you must plan for
• 6
  Grade family compared by use case

Imported mold steel is tool steel produced outside the country where it is used, sold under a mill or distributor brand. The main families are P-grade pre-hardened plastic mold steel, H-grade hot work steel, S-grade shock steel, and D-grade cold work steel. A tool room buys it for one reason: the mill certified the chemistry, the cleanliness, and the hardness, and that certificate travels with the block.

That certificate matters more than the flag on the crate. Two blocks of the same nominal grade can behave very differently if one has larger sulfide inclusions or a wider hardness spread across the section. For a cavity running 500,000 cycles, a 2 HRC spread from face to core shows up as uneven polish and early washout on the gate area.

The label does not guarantee a better steel. It guarantees a documented one. When a mold fails and the customer asks why, a traceable heat number and a mill test report end the argument fast. That paperwork is often the real product.

P-grade pre-hardened steel arrives at roughly 28–34 HRC and goes straight into the mill. You skip heat treatment, which removes the distortion risk and the two-week queue. It suits cores, cavities, and slides for commodity parts. It will not hold a mirror polish forever, and it wears faster at the gate.

H-grade hot work steel, typically 1.2343 or 1.2344 chemistry, is chosen when the cavity sees metal at 600 °C or more. Die casting dies, extrusion tooling, and hot forging inserts live here. It resists thermal fatigue cracking, which is the failure mode that kills a die after 80,000 shots.

S-grade shock steel and D-grade cold work steel cover the cold end. S-grade absorbs impact without chipping, so it suits shear edges and trim dies. D-grade runs 58–62 HRC after hardening and holds a sharp edge on punching and blanking work.

There is no universal grade. The failure mode decides the family, not the datasheet ranking.

• 1
  P-gradePre-hardened 28–34 HRC, no heat treat, plastic molds
• 2
  H-gradeHot work, thermal fatigue resistance, die casting
• 3
  S-gradeShock resistance, trim and shear edges
• 4
  D-grade58–62 HRC, wear resistance, punching and blanking

Pre-hardened P-grade at 30 HRC cuts cleanly with coated carbide. On our 3-axis and 4-axis mills we run 80–150 m/min surface speed, 0.1–0.2 mm per tooth feed, and 0.3–0.8 mm radial depth for roughing. The material is tough but not abrasive, so tool life is long and the finish comes off the cutter at Ra 1.6–3.2 μm.

Through-hardening grades are different. You machine them soft, at 180–220 HB, then send them to heat treatment and accept the movement. Plan 0.3–0.5 mm of stock on every critical surface for post-hardening grinding. If you cut to final size before hardening, you will scrap the block.

Hardened D-grade above 58 HRC cannot be milled with carbide at any sane cost. That is EDM and grinding work. Graphite electrode wear runs 0.1–0.3 % on roughing, and the recast layer on the finished surface needs a light stone or a fine EDM pass to remove.

Thin walls are the common trap. A 1.5 mm rib in a 32 HRC block will move 0.05–0.1 mm under cutter pressure. Rough it, let it rest, then semi-finish with a light radial pass.

Specify imported steel when the tool must run unattended, when a warranty claim would cost more than the block, or when the customer's drawing names the grade. Automotive and medical programs often freeze the material in the PPAP or design history file, and substituting it is not a free decision.

Skip it when the part is a bridge tool for a few thousand shots, when the geometry is simple, or when the mold will be rebuilt anyway. A domestic 1.2311 equivalent at 30 HRC will run a low-volume housing mold and cost far less. The honest test is whether a mold failure would stop a production line.

Price is not the whole comparison. Imported stock is usually ground square and pre-machined to size, so you buy less roughing time and less scrap. On a 400 × 500 mm cavity block, that saving can offset a meaningful share of the material premium.

Lead time cuts both ways. Standard pre-hardened blocks ship in days. Special chemistries and large sections can take weeks, and that is often the real reason a project slips.

Pre-hardened steel needs no treatment unless you weld it. If you weld a damaged gate or a cracked rib, preheat the area to 200–300 °C, use a matching filler, and stress relieve afterward. Skip the preheat and you get a hard, brittle zone that cracks on the next cycle.

Through-hardening grades need a controlled cycle: austenitize, quench, then temper twice. Double tempering is not optional on H-grade. A single temper leaves retained austenite that transforms later and changes the dimensions you just ground.

Expect 0.1–0.3 % linear movement on a normal block, and more on long, thin sections. Leave stock, harden, then grind. Vacuum hardening holds this tighter than atmosphere hardening because there is no gas quench turbulence on one face.

We send heat treatment to qualified vendors with furnace charts and hardness maps on request. We do not guess at the result, and we do not machine a hardened block back to size without knowing where it landed.