Effect of Tempering Temperature on Microstructure and Toughness of H13 Steel Weld Overlay Layer

Literature Overview

This 2015 paper published in Hot Working Technology by Bu Jianrong, Xu Junyan, and Xu Minyan from Zhejiang Industrial Vocational and Technical College investigates the influence of tempering temperature on the microstructure and toughness of H13 hot work tool steel weld overlay deposits. H13 (equivalent to Chinese 4Cr5MoSiV1) is a widely used hot work tool steel known for its excellent hot hardness, thermal fatigue resistance, and wear resistance at elevated temperatures. The study addresses a critical aspect of weld overlay engineering — post-weld heat treatment optimization.

Core Technical Content

H13 steel weld overlay layers are applied to hot work tooling such as die casting molds, forging dies, and extrusion dies that experience cyclic thermal loading, thermal fatigue, and abrasive wear at temperatures between 400–600°C. The as-welded microstructure of H13 overlay typically consists of martensite, retained austenite, and carbide precipitates. Post-weld tempering is essential to:

Reduce residual stress from welding.
Improve toughness and ductility.
Optimize the hardness-toughness balance.
Stabilize the microstructure against future thermal cycling.

Tempering Temperature Study Matrix

Tempering Temperature (°C)	Hardness (HRC)	Impact Toughness (J/cm²)	Microstructure	Application Suitability
500	52–54	8–12	Tempered martensite + fine carbides	High wear resistance required
550	48–50	12–18	Tempered martensite + medium carbides	Balanced wear/toughness
600	44–46	18–25	Tempered martensite + coarse carbides	High thermal fatigue resistance
650	40–42	22–30	Sorbite-like + coarse carbides	High impact loading

Microstructural Evolution

The microstructural evolution with increasing tempering temperature follows a well-established sequence:

500°C: Carbon atoms diffuse from supersaturated martensite to form fine, coherent carbides (M₇C₃, MC). The matrix retains high hardness with improved ductility compared to as-quenched condition.
550°C: Carbide coarsening begins. M₇C₃ transforms partially to M₂C. The tempered martensite laths become more distinct. Retained austenite decreases.
600°C: Significant carbide coarsening and spheroidization. M₂C becomes dominant. The microstructure approaches a tempered sorbite. Toughness is substantially improved at the expense of hardness.
650°C: Approaching second tempering. Carbides are coarse and well-dispersed. The matrix is fully tempered. Maximum toughness is achieved but hardness drops significantly.

Engineering Practice Integration

Heat Treatment Cycle Design

The recommended post-weld heat treatment for H13 overlay deposits is:

Solution treatment: 1050–1100°C, hold 2h, air cool (if required for stress relief)
Tempering: 550–600°C, hold 2–4h, air cool
Multi-stage tempering: For critical applications, two tempering cycles at 550°C and 600°C are recommended to eliminate retained austenite and stabilize carbides.

Application-Specific Optimization

Application	Recommended Tempering	Target Hardness	Rationale
Die casting molds	550°C	48–50 HRC	Balance of hot hardness and thermal fatigue
Forging dies	500°C	52–54 HRC	Maximum wear resistance at moderate temperature
Extrusion dies	600°C	44–46 HRC	High thermal fatigue and impact resistance
Hot shear blades	550°C	48–50 HRC	Combined abrasion and impact resistance

Defect and Failure Analysis

Failure Mode	Root Cause	Prevention
Thermal fatigue cracking	Insufficient tempering, high residual stress	Proper tempering at 550–600°C
Wear spalling	Excessive tempering, low hardness	Avoid > 650°C tempering
Cracking during cooling	Excessive cooling rate after tempering	Controlled cooling (≤ 100°C/h)
Soft spots	Uneven heat treatment, local overheating	Uniform furnace atmosphere, proper loading

Study Insights and Implications

This study provides a clear demonstration of the hardness-toughness trade-off in H13 weld overlay deposits. The key insight is that there is no single "optimal" tempering temperature — the selection must be application-specific, balancing wear resistance against thermal fatigue and impact resistance. A critical practical observation is that the weld overlay deposit microstructure differs from the base H13 steel due to dilution effects, segregation, and different solidification conditions. The weld metal typically contains higher carbon and alloy concentrations in the interdendritic regions, leading to higher retained austenite content and potentially lower toughness than the base material. Multi-stage tempering is strongly recommended to address this issue. Furthermore, the study highlights that the toughness improvement from tempering is not solely due to carbide precipitation — the reduction of residual stress and the transformation of retained austenite to stable martensite contribute significantly to the toughness gain. For engineers designing H13 overlay systems, the recommendation is to always perform hardness and impact testing on actual weld overlay coupons heat-treated under the same cycle as the production part, rather than relying solely on base metal data.