Heat Treatment Optimization for Spray Forming HSS

Spray forming has emerged as a powerful route for producing high speed steel (HSS) preforms with reduced segregation, refined microstructure, and improved isotropy compared to conventional casting. However, to fully unlock the performance potential of spray forming HSS, heat treatment must be carefully optimized. This article explores the key metallurgical objectives, critical parameters, and practical strategies for tailoring heat treatment to spray formed HSS.

Why Spray Forming HSS Requires a Different Heat Treatment Approach

Unlike cast HSS, spray forming HSS typically features:

• Finer and more uniformly distributed carbides
• Lower macro-segregation and fewer coarse eutectic networks
• Residual micro-porosity depending on deposition parameters
• Higher supersaturation of alloying elements in the matrix

These distinctions directly influence phase transformations, carbide precipitation behavior, and diffusion kinetics, meaning heat treatment schedules used for cast or PM-HSS may not yield optimal results for spray formed material.

Heat Treatment Goals for Spray Forming HSS

The ideal heat treatment strategy should:

1. Preserve the refined carbide dispersion while eliminating unstable carbides
2. Maximize secondary hardening response
3. Reduce retained austenite without excessive grain growth
4. Promote matrix homogenization and relieve residual stress
5. Avoid cracking at prior pore boundaries

Optimized Heat Treatment Workflow

A typical optimized cycle includes the following stages:

1. Pre-Annealing 

• Temperature: 850–900 °C
• Soak Time: 1.5–3 h 
• Cooling: Furnace cool to < 500 °C

Purpose:

• Reduce alloying element supersaturation gradients
• Relieve internal stress from rapid droplet solidification
• Stabilize M₂C/M₆C transition pathways prior to austenitization

2. Austenitization

• Temperature: 1,180–1,230 °C (grade-dependent)
• Soak Time: 3–6 min/mm of section thickness
• Atmosphere: Vacuum or inert gas preferred to limit decarburization
• Quenching: High-pressure gas quench or oil quench for large sections

Optimization Notes:

• Spray forming HSS benefits from slightly lower austenitization temperature than PM-HSS due to faster dissolution kinetics
• Overheating risks carbide coarsening and grain boundary cracking
• Aim for partial MC/M₆C dissolution while avoiding full carbide meltback

3. Cryogenic Treatment

• Temperature: −120 to −196 °C
• Duration: 1–3 h

Benefits:

• Converts unstable retained austenite
• Enhances dimensional stability
• Improves tempering efficiency

4. Tempering (Secondary Hardening Optimization)

• Temperature: 540–580 °C
• Cycles: 2–3 tempers, 1–2 h each
• Cooling: Air cool between tempers

Critical Effects:

• Promotes M₂C → M₆C transition
• Triggers strong secondary hardening peak
• Maintains carbide fineness if temperature is controlled

Tip: Many spray formed grades show peak hardness at ~560 °C, but the exact peak should be validated experimentally via hardness-tempering curves.

Key Optimization Strategies

Retained Austenite Reduction

• Use cryo + multi-stage tempering
• Avoid excessive quench rates in large sections that form transformation stress at pore sites

Grain Size Management

• Limit austenitization temperature
• Avoid long soak times above 1,230 °C
• Utilize rapid quench methods where possible

Cracking Prevention

• Pre-anneal stress relief is essential
• Use uniform heating and avoid steep thermal gradients
• For high porosity preforms, consider slower initial quench stage before full cooling

Performance Validation Checklist

After heat treatment, spray forming HSS should be tested for:

1. Hardness response and secondary hardening peak
2. Dimensional stability
3. Impact and wear resistance
4. Retained austenite fraction (XRD recommended)
5. Microstructure (carbide size, morphology, distribution)

With an optimized thermal schedule, spray forming HSS can deliver exceptional tooling performance, combining the microstructural advantages of PM-HSS with the scalability and cost efficiency of spray deposition.