Why Thick Mold Steel Blocks Miss Core Hardness - China

A buyer orders a 600 × 600 × 300 mm block of 1.2311, pre-hardened to 30–34 HRC.

Six weeks later the block arrives, the mill certificate reads 31.5 HRC, the faces are machined clean, and a test hole 150 mm deep returns 27 HRC.

The emails that follow are usually framed as a quality dispute.

In most cases it is not one.

The heat was not wrong and the certificate was not falsified.

The purchase order asked for a hardness without ever saying where that hardness had to exist, and hardness in a thick section is not a single number — it is a curve that falls as you move inward.

This is the most common misunderstanding in the trade, and it costs both sides money: the buyer because a block has to be reworked, replaced or accepted with a softer core than the tool design assumed, and the mill because a perfectly sound heat gets treated as defective.

The fix is not a better grade.

It is a better specification, built on an understanding of what hardenability actually is.

Hardness and hardenability are two different properties Hardness is a local measurement.

A diamond or tungsten ball is pressed into a prepared surface at a stated load, and the size of the impression is converted to a Rockwell, Brinell or Vickers number.

It tells you what that one point, at that one depth, resists.

Hardenability is a property of the steel itself: its capacity to form martensite at depth when cooled at a given rate.

It is fixed by chemistry — carbon, manganese, chromium, molybdenum, nickel — together with prior austenite grain size, and it does not change when you machine the block.

Confusing the two is what produces the scenario above.

Almost any steel can be made hard in a thin section.

A 0.4% carbon steel quenched as a 10 mm plate will form martensite through its full thickness and temper to 50 HRC without difficulty.

The same steel quenched as a 600 mm block will form martensite at the surface and something else entirely at the centre, because the centre simply does not cool fast enough.

Hardness at the surface is easy.

Hardness at depth is hardenability, and hardenability is bought with alloy content.

The metallurgical reason is straightforward.

Austenite must be cooled past the temperature range where it wants to decompose into ferrite, pearlite and bainite.

If it is cooled quickly enough, it misses those reactions and transforms to martensite by a diffusionless shear, which is what gives the high hardness.

If it is cooled slowly, the austenite has time to transform into softer products before it ever reaches the martensite start temperature, and no subsequent tempering can recover the difference.

A commonly used definition of "through-hardened" is that the core contains at least 50% martensite before tempering.

For demanding tooling, 80–90% is the more realistic target.

That distinction matters commercially, because a block that is 55% martensite at the core is technically through-hardened and will still read several points softer than the surface after tempering.

The Jominy end-quench test, in plain terms Hardenability is measured with the Jominy end-quench test, standardised as ISO 642 and ASTM A255 - A round bar 25 mm in diameter and 100 mm long is austenitised at the grade's normal hardening temperature, then quenched by a controlled water jet playing on one flat end only.

Because cooling is fastest at the quenched end and slows progressively along the bar, the specimen contains a continuous spectrum of cooling rates in a single piece.

After quenching, a flat is ground along the side and hardness is measured at 1.5 mm intervals from the quenched end.

The result is a curve: hardness in HRC against distance from the quenched end in millimetres.

That curve is the grade's hardenability fingerprint, and it is far more informative than a single hardness number.

Reading the curve

Company Contact:

• Posted By: Hubei Keyspark Special Steel Co., Ltd.

• Phone: +8615024056480

• Address: 30 Huahu Avenue, Huangshigang District, Huangshi , Hubei , China

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• Website: https://keysparksteel.com

• Business Description: Hubei Keyspark Special Steel Co., Ltd. is an integrated special steel manufacturer in Huangshi, Hubei, China.
  
  Melting, ladle refining, vacuum degassing, ESR, forging and heat treatment are all in house.
  
  Products: plastic mold steel (1.2311/P20, 1.2312, 1.2316, 1.2738), cold work steel (1.2083, 1.2379/D2, DC53), hot work steel (1.2344/H13, 1.2714), high speed steel, alloy and stainless steel.
  
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