CLADDING TECHNOLOGY SHANXI CO., LTDEffects of Tempering Temperature on Microstructure and Toughness of H13 Steel Weld Overlay Deposits
Literature Overview
H13 (4Cr5MoSiV1 in Chinese classification) hot work tool steel is widely used for hot work dies, forging dies, and extrusion tooling. Weld overlay of H13 material on base components is essential for die repair and surface hardening applications. This study focuses specifically on the tempering temperature effects on the weld overlay deposit microstructure and, critically, on toughness — a property that is often sacrificed in pursuit of hardness in conventional overlay practice.
Material Background and Challenge
H13 steel achieves its characteristic hot work performance through a combination of high-temperature strength, thermal fatigue resistance, and adequate toughness. The standard H13 tempering temperature range is 500–650 °C, but weld overlay deposits present unique challenges due to their non-equilibrium microstructure, residual stress state, and potential for retained austenite.
Tempering Parameters and Microstructural Response
| Tempering Temperature (°C) | Matrix Structure | Carbide Type | Hardness (HV) | Impact Energy (J/cm²) | Retained Austenite (%) |
|---|---|---|---|---|---|
| As-quenched | Martensite + retained austenite | None (undissolved) | 580–620 | 8–12 | 15–20 |
| 400 | Tempered martensite | Fine M23C6 | 550–580 | 10–14 | 12–16 |
| 500 | Tempered martensite | M6C + M23C6 | 480–520 | 18–24 | 8–12 |
| 550 | Tempered martensite | M6C dominant | 450–490 | 28–35 | 5–8 |
| 600 | Tempered martensite + some ferrite | M6C + coarse M23C6 | 420–460 | 35–42 | 3–5 |
| 650 | Tempered martensite + ferrite | Coarse M6C | 380–420 | 38–45 | 2–4 |
Key Technical Findings
Hardness-Toughness Trade-off
The study clearly demonstrates the classic hardness-toughness trade-off in tempering, but with important nuances specific to weld overlay deposits. The optimal tempering temperature for H13 overlay deposits depends on the service application:
- High abrasion service (mining, crushing): 500–550 °C provides hardness of 480–520 HV with acceptable toughness of 18–35 J/cm².
- Thermal fatigue service (hot dies, casting molds): 600–650 °C maximizes thermal fatigue resistance through improved thermal conductivity and reduced residual stress.
- Impact-abrasion combined service: 550 °C offers the best compromise with 450–490 HV and 28–35 J/cm².
Retained Austenite Behavior
A critical finding is that tempering temperature directly controls retained austenite stability in H13 overlay deposits. Unlike base metal H13 which is typically fully tempered to eliminate retained austenite, overlay deposits retain higher austenite levels due to the welding thermal cycle. Tempering at 550–600 °C effectively stabilizes and transforms most retained austenite to martensite or bainite, eliminating the risk of delayed austenite transformation under service loading.
Residual Stress Reduction
| Tempering Temperature (°C) | Residual Stress (MPa) | Reduction from As-Welded (%) |
|---|---|---|
| As-welded | 450–520 | — |
| 400 | 380–440 | 15–20 |
| 500 | 280–340 | 35–45 |
| 550 | 200–260 | 50–60 |
| 600 | 150–200 | 60–70 |
| 650 | 100–150 | 70–80 |
The substantial residual stress reduction achieved at 550–650 °C is particularly important for overlay deposits on thick base plates where differential cooling creates complex stress states.
Defect Analysis and Countermeasures
| Defect Type | Cause | Prevention/Countermeasure |
|---|---|---|
| Hot cracking | Low-ductility phases in grain boundaries | Preheat 200–300 °C, control heat input |
| Cold cracking | Hydrogen embrittlement in martensitic microstructure | Post-weld bake 200 °C for 2h before tempering |
| Excessive porosity | Gas entrapment in thick deposits | Use low-hydrogen consumables, proper shielding |
| Undercut at stirred edges | Excessive stirring penetration | Limit stirring depth to 1.5–2.0 mm |
| Cracking during tempering | Thermal stresses in thick deposits | Slow furnace heating rate (<100 °C/h) |
Engineering Practice Integration
For die repair operations where H13 overlay is applied to damaged or worn die surfaces, the recommended process sequence is:
- Preheat base material to 200–300 °C to prevent base metal cracking.
- Deposit H13 overlay using low-hydrogen consumables (E70T-8 or equivalent with modified chemistry).
- Post-weld hydrogen bake at 200 °C for 2 hours to eliminate diffusion hydrogen.
- Tempering at 550 °C for 2 hours (air cool from furnace) for general service.
- Second temper at same temperature if deposit thickness exceeds 10 mm.
This sequence balances toughness requirements with hardness retention and minimizes the risk of thermal cracking during the tempering operation itself.
Study Insights and Implications
This research challenges the common industry practice of tempering H13 overlays at high temperatures (600–650 °C) to maximize toughness at the expense of hardness. The study demonstrates that 550 °C tempering provides a superior balance for most hot work applications, maintaining adequate hardness (450–490 HV) while achieving near-maximum toughness (28–35 J/cm²). More importantly, the study highlights that retained austenite control through proper tempering is essential for long-term service reliability — untransformed retained austenite can undergo stress-induced transformation during die service, leading to unexpected dimensional changes and potential catastrophic failure. Engineers responsible for die repair specifications should incorporate these findings into their welding procedure qualifications and post-weld treatment protocols.