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How to improve D2 Metal Properties?

D2 steel offers nearly unmatched hardness and wear resistance among similarly‑priced steels, but it has weaknesses of low toughness and poor corrosion resistance. This article covers five processes: raw material smelting, forging, heat treatment, cryogenic treatment and surface coating, and shows how to boost the Properties of D2 metal from the source.

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Smelting and Refining of D2 Metal

The purity of D2 metal sets its upper performance limit. If oxygen content goes higher, more inclusions will form, and toughness will drop. The whole smelting process starts with EAF. Next, it goes through LF plus VD refining. After that, ESR is applied to further raise material purity.

EAF (Electric Arc Furnace Primary Melting)

EAF is the starting point for steel smelting. Graphite electrodes produce arcs above 3000°C. These arcs melt scrap steel and alloy materials like ferro‑chromium, ferro‑molybdenum and ferro‑vanadium into liquid metal.

3 Main Core Tasks of EAF:

  1. Melting and alloying: It roughly tunes chemical composition to D2 target ranges: C 1.4‑1.6%, Cr 11‑13%, Mo 0.7‑1.2%, V ≤1.0%.
  2. Dephosphorization: Slag‑making and oxygen‑blowing work to take away phosphorus. Phosphorus content can stay below 0.015% before EAF tapping. Under D2 patented process, the value can fall to 0.009%.
  3. Decarburization and temperature increase: Oxygen blowing removes carbon. At the same time, oxidation reactions give off heat to raise temperature. The tapping temperature is controlled near 1620°C.

LF+VD Refining

After tapping from EAF, molten steel goes through LF (Ladle Furnace) and VD (Vacuum Degassing) processes. LF removes sulfur and phosphorus, while VD eliminates hydrogen and oxygen. The LF+VD refining process can keep the oxygen content ≤16 ppm. Lower oxygen content means fewer inclusions, and brings higher toughness plus longer fatigue life for D2 tool steel.

Electroslag Remelting (ESR)

D2 tool steel produced by regular electric furnace smelting has coarse carbides with uneven distribution. ESR (Electroslag Remelting) is a key method to improve the quality of D2 metal. ESR uses slag refining to further cut oxygen content and non‑metallic inclusions. It makes the structure of D2 metal denser and its carbides more uniform.

Electroslag Remelting (ESR)

The surface condition of D2 steel electrode rods directly affects the oxygen content of D2 metal. ESR ingots remelted from black‑skin electrode rods show 25 % higher oxygen content compared with machined electrodes. For high‑quality D2 material, ESR electrode rods can choose machined finishing or sand‑grinding for surface treatment.

Forging of D2 Metal

In the as‑cast state, D2 features coarse network‑distributed carbides. Forging serves to break up this network structure and make carbides evenly dispersed.

Forging Ratio

The forging ratio (cross‑section area before forging / cross‑section area after forging) is a key parameter that influences the carbide uniformity of D2 metal. Studies show that within the forging ratio range of 2‑8, the rating of eutectic carbide non‑uniformity for AISI D2 drops as the forging ratio rises. A higher forging ratio delivers finer and more uniform carbides. Normally, Steel D2 requires a forging ratio above 3:1, and some high‑end products demand over 5:1.

Three‑Upsetting‑Three‑Drawing

Single upsetting‑drawing cannot fully break the coarse carbides inside D2 metal. Keyspark Steel adopts the three‑upsetting‑three‑drawing process, which means three times of upsetting and three times of drawing carried out alternately. Its main goal is to break carbides in steel and remove their orientation. In this way, forgings can meet requirements for impact resistance, hardness and wear resistance. Every steel ingot must go through three‑upsetting‑three‑drawing, so the as‑cast structure at the ingot core can be fully broken.

Typical process parameters

Initial forging temperature 1150‑1180°C, finish forging temperature ≥800°C. The forging ratio for each upsetting is 2‑2.2, and the forging ratio for each drawing is 2‑2.2. The reduction per upsetting is 10‑15 %, and the reduction per drawing is 10‑15 %. After three cycles of upsetting‑drawing, internal carbides of steel are fully broken and orientation is eliminated. The performance of this tool steel gets improved.

Ingot Size

Under the same forging ratio, the eutectic carbide non‑uniformity of D2 metal rises as the ingot size increases. Large‑size ingots are harder to forge, and their carbides are more difficult to break. Therefore, small‑batch and high‑quality Material D2 achieves better control by using small‑size ingots.

Annealing of D2 Metal

After forging, D2 steel features hard internal microstructure and high internal stress. Direct machining will damage cutting tools. Annealing softens the material, relieves internal stress and makes carbides evenly distributed. It brings more sufficient austenitization during subsequent quenching, with more uniform hardness and better toughness. As a result, smooth turning, milling and drilling can be realized.

Conventional Annealing (Stress‑Relief Annealing)

It aims to relieve residual internal stress generated from forging, rolling or rough machining and lower material hardness. The process requires heating the material to 600‑650 °C and holding it for 2 hours. Next, cool it inside the furnace until the temperature drops to 500 °C, then take it out for air cooling. This method is applied after rough workpiece machining to eliminate cutting stress and avoid deformation in the following heat‑treatment steps.
 
 

Spheroidizing Annealing (Core Annealing Process for D2)

Spheroidizing annealing is the core annealing process for D2. As a ledeburite steel, D2 produces coarse network‑shaped carbides under the as‑cast condition. Spheroidizing annealing does not simply soften the material. Instead, it converts coarse plate‑shaped carbides into fine and round spherical carbides to prepare the microstructure for subsequent quenching. The standard spheroidizing annealing process heats the steel to 860 °C and holds it for 1 hour, followed by isothermal spheroidization at 740 °C for 6 hours. After furnace cooling to 500 °C, the workpiece is taken out for air cooling. The measured Ac1 temperature of this steel is 827 °C, so the annealing temperature should be kept near Ac1.

Heat Treatment of D2 Steel

Tempering

D2 metal must adopt double tempering. Single tempering can only remove about 70 % of internal stress. Double tempering can completely relieve stress and stabilize dimensions.
  1. Low‑temperature tempering (180‑230°C): D2 steel reaches 60‑62 HRC, it fits scenarios of high wear resistance and low impact.
  2. High‑temperature tempering (500‑560°C): AISI D2 gains hardness ≥58 HRC, with lower internal stress and more stable dimensions.

The heat treatment of D2 tool steel determines its final combination of hardness and toughness.

Quenching Temperature

The quenching temperature for D2 metal normally ranges from 1000‑1040°C. Higher temperature allows alloy elements to dissolve more completely, and brings higher hardness. However, too high temperature will cause coarse grains and reduce toughness. It reaches its optimal hardness when quenched at 1010‑1040°C. If you want to improve the toughness of AISI D2 steel, you can set the quenching temperature at the lower limit of 1000‑1020°C. It sacrifices 1‑2 HRC hardness for better impact resistance.

Tempering

D2 cold work tool steel must adopt double tempering. Single tempering can only remove about 70 % of internal stress. Double tempering can completely relieve stress and stabilize dimensions.
  1. Low‑temperature tempering (180‑230°C): D2 metal reaches 60‑62 HRC, it fits scenarios of high wear resistance and low impact.
  2. High‑temperature tempering (500‑560°C): D2 metal gains hardness ≥58 HRC, with lower internal stress and more stable dimensions.

Cryogenic Treatment of D2 Tool Steel

Cryogenic treatment is one of the most cost‑effective ways to improve the performance of D2 metal.

Principle

After D2 metal is quenched, 10‑20 % austenite (retained austenite) remains in its microstructure. Such retained austenite is unstable. It slowly turns into martensite during service and causes dimensional change. Cryogenic treatment with liquid nitrogen at ‑196 °C converts retained austenite into martensite and precipitates fine carbides at the same time.

Function

After quenching and tempering, cryogenic treatment at ‑196 °C for 2 hours raises the surface hardness of D2 metal by 5.38 % compared with samples without cryogenic treatment. Its wear volume drops by 68.89 % at 20 °C and drops by 77.74 % at 200 °C. Its impact toughness increases by 40.80 %.

FAQ:

Q1: What is the suitable forging ratio for D2 metal?

A: Within the forging ratio range of 2‑8, carbide non‑uniformity decreases as the forging ratio increases. High‑quality D2 metal generally requires a forging ratio above 3:1, combined with the three‑upsetting‑three‑drawing process.

A: D2 is ledeburite steel. Its carbides show coarse network distribution under the as‑cast state. Spheroidizing annealing transforms these coarse plate‑shaped carbides into fine and round spherical carbides. It reduces hardness, improves machinability, and prepares microstructure for the subsequent quenching process.

A: The LF+VD refining process can control the oxygen content of D2 metal ≤16 ppm. If the value goes beyond this limit, more inclusions will form and toughness will decline.

A: It depends on requirements. It is unnecessary for general applications. Cryogenic treatment is recommended when high dimensional stability and high toughness are required. It can raise hardness by over 5 % and improve toughness by 68 %.

A: Isothermal quenching holds the steel at 270 °C for 6 hours. It obtains mixed microstructure of lower bainite plus martensite. It reaches hardness of 61.1 HRC and toughness of 56 J/cm², delivering better toughness than conventional quenching.

Conclusion

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