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D2 Die Steel - Causes and Improvements of Hot‑Rolled Surface Cracking

Surface cracking of D2 Die Steel during hot rolling is a common quality problem in the industry. Starting from metallurgical root causes, this article explains how cracks form and how to control them in forging and rolling processes.

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D2 Die Steel
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Why does D2 Die Steel tend to crack?

High‑Alloy of D2 Material

AISI D2 is a high‑carbon high‑chromium cold‑work die steel. It contains C: 1.40‑1.60% and Cr: 11.00‑13.00%, plus strong carbide‑forming elements such as molybdenum and vanadium. These alloy elements give D2 steel great wear resistance and compression strength, yet they also bring one side effect: its hot plasticity drops greatly. It has a very narrow hot‑working temperature window, only around 900‑1170°C. Small mistakes will cause surface cracks or even edge cracks during hot rolling.

Massive Carbide Effect

The root cause for low hot plasticity of D2 tool steel lies in large amounts of primary carbides and coarse eutectic structure networks. Hard and brittle coarse eutectic carbides cannot flow together with the steel matrix under hot deformation. When the matrix bears pulling force, the boundaries between carbides and the matrix become stress‑concentration spots. Cracks start here and spread along carbide bands.

Inclusion Caused by Carbide

Carbides also easily form on inclusions, which further raises cracking risk. To fundamentally improve the hot‑rolled surface quality of D2 die steel, the carbide problem must be solved first.

Types and hazards of surface cracking in D2 Die Steel

D2 die steel mainly has two crack forms during hot rolling: edge cracking and surface cracking.

Edge Cracking

It occurs at the edge area of rolled plates or flat bars and distributes longitudinally. Under the same chemical composition, the hot‑rolling yield decreases significantly with the increase of product profile aspect ratio. Edge cracking is the most common and hardest‑to‑control defect in D2 material hot rolling.

Surface cracking

It appears on the surface of rolled products with various forms. In severe cases, it may extend inward.
 

Hazards

These cracks not only reduce AISI D2 steel yield and increase grinding repair costs. More seriously, tiny surface cracks may expand in subsequent heat treatment or service, and become the source of early failure for punches and dies. For buyers, hot‑rolled cracks often mean deeper internal metallurgical defects, such as carbide segregation and excessive inclusions.

Cracking Reasons of D2 Material

Material Factors of D2 Die Steel

Reduced Hot Plasticity Caused By High‑alloy Content

Elements such as Cr, Mo and V added in AISI D2 steel form large amounts of carbides including MC, M₇C₃, M₂₃C₆. During hot working, carbides strongly block the movement of dislocations and grain boundaries. As deformation continues, holes easily form around carbides due to decohesion and/or particle fracture. Combined with wedge‑shaped cracks at triple grain boundaries, these holes cause a sharp drop in hot plasticity of the tool steel. It should be noted that this effect also appears in the medium‑temperature range of 1000‑1100°C, not only in low‑temperature range.
 

Banded Segregation Of Carbides

If large numbers of edge cracks appear on your purchased hot‑rolled D2 cold‑work die steel, please note the cracks form during rolling and spread along banded eutectic carbides. This is typical eutectic carbide cracking. Zones with band‑clustered carbides become high‑risk areas for crack growth. Cracks extend rapidly along carbide bands and finally bring about edge cracking.
 

Inducing Effect Of Inclusions

Carbides easily precipitate on inclusions such as oxides. These precipitation spots become preferred positions for micro‑crack generation. Interfaces between inclusions and matrix have weak bonding force. Holes form easily by decohesion under hot‑deformation stress and further trigger cracks.
 

Process Factors Temperature Control And Processing Parameters of AISI D2

Strong Correlation Between Chromium Content And Soaking Temperature

Cr content has obvious influence on the soaking process of D2 steel, and closely relates to hot workability at upper and lower limits of hot‑working temperature range. It means D2 from different heats (Cr content varies within 11‑13%) may need adjusted soaking temperature to get optimum hot workability. Laboratory hot compression tests are used to find the best soaking temperature and widen the safe hot‑working temperature window.
 

Temperature‑window Control For Forging And Rolling

The start hot‑working temperature for D2 cold work steel is 900‑1040°C, and finish‑forging temperature shall not be lower than 925°C. Forging heating shall be slow and uniform, with soaking at 1850‑1950°F (approx.1010‑1065°C). Working must stop when temperature falls below 1700°F (926°C).

 

Hot rolling follows similar temperature rules as forging. Deformation must finish within this narrow window. Too low temperature brings sharp drop of hot plasticity and much higher cracking risk. Too high temperature may cause coarse grains and over‑burning. Billet temperature drops fast during rolling. Poor control easily causes quality problems such as tensile cracking.

 

Influence Of Profile Aspect Ratio

With identical chemical composition, hot‑rolling yield drops greatly when product profile aspect ratio rises. Edge zones bear heavier stress concentration in rolling. Larger aspect ratio leads to stronger edge stress concentration and higher chance of crack initiation.
 

Control Of Harmful Elements

Besides standard requirement P ≤0.030%, practical rolling needs P kept below 0.022% to avoid semi‑finished product defects. Apart from carbon and carbide‑forming elements, elements such as Mn, S, Cu, Sn and Al also exert notable influence on surface‑crack formation.

Improvement Measures of D2 Die Steel

Smelting Stage of D2 Tool Steel

Improve Electro‑Slag Remelting (ESR) Process

Electro‑slag remelting is a key refining step in D2 production. Optimizing the ESR process can effectively lower carbide segregation and inclusion content. Optimizing slag system, melting speed control and electrode quality can greatly improve ingot purity and microstructure uniformity.

Strengthen EAF+LF+VD Smelting Control

Strengthen EAF de‑phosphorization, LF refining, VD degassing and other steel‑making steps to boost the cleanliness of D2 die steel molten steel and directly improve the hot‑rollability of ingots. The hot‑rolling yield of D2 steel can rise to 73.9%. Specific measures:

  • Fully remove phosphorus during EAF smelting, keep P below 0.022%
  • Use white slag practice for LF refining, and keep slag‑holding time ≥30 min
  • Keep VD vacuum degree ≤1 mbar with holding time ≥15 min
  • Carry out soft Ar‑gas blowing for 10‑15 min before pouring, and set pouring temperature at 1460‑1470°C

Precise Control Of Chemical Composition

  • Keeping chemical element concentrations near precise target values or tighter ratios can effectively reduce surface cracking. It means:
  • Strictly limit heat‑to‑heat fluctuation of key elements such as C, Cr, Mo, V
  • Trace elements including Mn, S, Cu, Sn, Al shall also be under control
  • P content shall be kept below 0.022%

Homogenization Treatment Stage Microstructure Preparation

High‑temperature Homogenization Treatment Of Ingot

Heat the D2 steel ingot to a temperature slightly below the solidus line and hold for a long time. This operation can eliminate crystal segregation and regional segregation formed during ingot solidification, and make chemical composition and microstructure uniform.

Key Parameters:
  • Diffusion temperature: 1150‑1200°C
  • Diffusion time: 15‑17 h

Optimized Heating And Forging Process

  • To prevent banded carbide cracking, the heating and forging process shall be improved:
  • Pre‑heat slowly to 700°C, then raise temperature rapidly to 900‑1040°C
  • Finish‑forging temperature shall not be lower than 925°C
  • Re‑heat repeatedly as required to keep proper forging temperature
  • Slow cooling after forging

Rolling Stage Process Optimization

Control Of Deformation Amount And Rolling Speed

Keyspark Steel controls different deformation amounts and rolling speeds during D2 ingot rolling to break large‑size carbides and reduce uneven distribution of eutectic carbides. This prevents defects such as surface cracks and internal cracks in rolling.

  • Step 1 — Small‑deformation Blooming Stage: Apply small deformation of 3‑6% per pass for 4 rolling passes. This step aims to get fine grains inside the ingot at first, prepare for later heavy deformation and avoid cracks caused by excessive initial deformation.
  • Step 2 — Heavy‑deformation Breaking Stage: Apply heavy deformation of 20‑30% per pass for 2 rolling passes. The main goal of this step is to break coarse eutectic carbide networks by heavy deformation and improve carbide distribution uniformity.
  • Step 3 — Finishing Rolling Forming Stage: Apply deformation of 5‑12% per pass for 9‑11 rolling passes. Roll speed is kept at 40‑60 r/min for the pass before final product and final pass, to guarantee surface quality and dimensional accuracy.
Finish‑rolling temperature shall not be lower than 900°C. During rolling, avoid cooling‑water splashing onto the billet. Fast surface temperature drop from water splash will cause rolling cracks.

 

Post‑rolling Annealing And Ultrasonic Testing of D2 Die Steel

Carry out hot‑charge annealing at 850‑900 °C after rolling. Finished products shall be inspected by ultrasonic testing following Group 3 of SEP1921, and reach E/e quality grade. It means the material has zero surface defects and its internal quality meets high‑level international standards. For example, one patent produces 310 mm×100 mm flat bars with zero surface defects and UT grade SEP1921 E/e.

What To Focus On When Purchasing AISI D2

  1. ESR Process Capacity: Electro‑slag remelting is the key procedure to control carbide segregation and inclusions
  2. High‑temperature Homogenization Treatment Capacity: Whether it has homogenization treatment capacity of 1150‑1200°C×15‑17h
  3. Rolling Process Control Capacity: Whether it has multi‑stage deformation control and precise temperature control capacity
  4. Ultrasonic Test Report of D2 material: Whether Group 3 ultrasonic testing is carried out according to SEP1921 standard after rolling, and whether the quality grade reaches E/e
  5. Smelting Control Level: Whether P is controlled below 0.030%, and whether it has complete EAF+LF+VD refining capacity

FAQ of D2 Die Steel

Q1: What is the root cause of hot‑rolled surface cracking for D2 die steel?

A: The root lies in the alloy design of D2 die steel. C: 1.40‑1.60%, Cr: 11.00‑13.00% give AISI D2 steel an extremely narrow hot‑working window of only 900‑1170°C. Coarse eutectic carbides formed in casting state gather in bands. During hot deformation, interfaces between carbides and matrix cannot deform in harmony. Stress builds up and cracks form along carbide bands.

A: Heating shall be slow and uniform. D2 die steel soaks at 1010‑1065°C (1850‑1950°F). The finish‑forging temperature shall not be lower than 925°C (1700°F). Below this temperature, hot plasticity of D2 material drops sharply and deformation resistance rises fast. Continuous forging will easily produce cracks.

A: D2 die steel has poor thermal conductivity. Cooling‑water spray causes sudden local temperature drop and brings thermal stress. This thermal stress adds to rolling stress. Once beyond material bearing limit, surface cracking will occur.

A: ESR (Electro‑Slag Remelting) reduces inclusions and carbide segregation in D2 die steel. For smelting, keep P below 0.030% and apply full‑process EAF+LF+VD refining. None of these steps can be skipped. With proper smelting work, hot‑rolling yield of D2 material can reach 73.9%.

A: Shallow surface cracks can be removed by peeling or turning. But internal defects such as broken coarse carbides can only be found by ultrasonic testing and cannot be removed by machining. When purchasing D2 die steel, ask for ultrasonic test report under SEP1921 standard. The quality grade shall reach D/d or E/e.

Conclusion

AISI D2 hot‑rolled surface cracking is a chain‑type problem running from chemical composition→carbide morphology→hot‑working parameters→final quality.
 
  1. Root causes lie in the material: D2’s high‑alloy content results in poor hot plasticity, with a hot‑working temperature window of only about 900‑1170°C. Banded segregation of carbides and inclusions are direct triggers for cracks.
  2. The key lies in the process: chromium content affects soaking temperature selection; the temperature window for forging and rolling is extremely narrow, finish‑forging temperature must not be lower than 925°C; excessive profile aspect ratio causes edge stress concentration; P content must be controlled below 0.030% at minimum.
  3. Improvement demands systematic measures: every link shall be precisely controlled, from ESR refining, precise chemical composition control, high‑temperature homogenization treatment (1150‑1200°C, 15‑17 h), multi‑stage deformation control (small deformation 3‑6% → heavy deformation 20‑30% → finish rolling 5‑12%), to post‑rolling acceptance via ultrasonic testing (SEP1921 E/e grade).

More D2 Tool Steel Resources

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📧 Email: Sales@keysparksteel.com

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