CLADDING TECHNOLOGY SHANXI CO., LTDEffect of Welding Current on Microstructure and Cracking of Plasma Weld Overlay H13 Steel
Literature Overview
H13 (AISI 4140 equivalent) is a hot-work tool steel widely used in die casting, forging, and extrusion applications where resistance to thermal fatigue and abrasive wear is required. Plasma transferred arc (PTA) weld overlay is the preferred method for restoring worn H13 components or depositing wear-resistant surfaces on tooling. This study examines the influence of welding current on the microstructure, hardness distribution, and cracking behavior of PTA-deposited H13 overlay layers, providing critical guidance for process parameter selection.
Microstructural Evolution with Welding Current
The welding current in PTA welding directly controls the heat input and cooling rate, which in turn govern the phase transformation behavior of the deposited H13 alloy. H13 is a medium-carbon, chromium-molybdenum tool steel with a nominal composition of 0.38-0.45% C, 4.95-5.50% Cr, 1.40-1.65% Mo, and 0.20-0.40% V. The microstructure is highly sensitive to cooling rate, with martensite forming at high cooling rates and bainite or tempered martensite appearing at lower cooling rates.
| Welding Current (A) | Heat Input (kJ/mm) | Cooling Rate (°C/s) | Dominant Microstructure | Hardness (HV) | Crack Length (mm) |
|---|---|---|---|---|---|
| 100 | 0.8 | 80-120 | Fine martensite + retained austenite | 520-560 | 25-35 |
| 150 | 1.2 | 40-70 | Martensite + bainite + retained austenite | 480-520 | 10-18 |
| 200 | 1.8 | 20-40 | Bainite + tempered martensite | 430-470 | 2-8 |
| 250 | 2.5 | 10-25 | Coarse bainite + spheroidized carbides | 400-440 | 0-3 |
At low welding currents (100-125 A), the high cooling rate produces a fine martensitic microstructure with high hardness but also high residual stress and significant cracking tendency. The retained austenite fraction increases with decreasing current, as the rapid cooling traps austenite that would otherwise transform to martensite. At high welding currents (225-250 A), the lower cooling rate allows more complete transformation and the formation of coarser bainitic structures, resulting in lower hardness but substantially reduced cracking.
Cracking Mechanisms and Countermeasures
Cracking in PTA-deposited H13 overlay layers occurs through two primary mechanisms: hot cracking during solidification and cold cracking during cooling below 500°C. Hot cracking is associated with the presence of low-melting-point phases at grain boundaries, particularly manganese sulfide inclusions that form liquid films during solidification. Cold cracking is driven by the combination of martensitic transformation, hydrogen embrittlement, and residual stress.
The literature identifies several effective countermeasures:
- Preheating the base material to 250-400°C to reduce the cooling rate and suppress martensitic transformation in the dilution zone
- Applying a low-carbon or austenitic transition layer before the H13 overlay to reduce the carbon activity at the interface
- Using a multi-pass approach with interpass temperature control of 200-300°C
- Post-weld stress relief at 600-650°C for 2-4 hours to relieve residual stresses and temper the martensite
- Controlling the sulfur and phosphorus content in the filler material to below 0.015% and 0.020% respectively
Process Optimization Recommendations
The optimal welding current for PTA H13 overlay depends on the specific application requirements. For applications where maximum hardness is required and the component geometry permits preheating and post-weld heat treatment, a current of 125-150 A with preheating to 300°C provides an acceptable balance of hardness (480-520 HV) and crack resistance. For applications where cracking must be minimized at all costs, such as large-diameter dies or thick-section components, a current of 200-225 A with preheating to 400°C and post-weld stress relief is recommended, accepting a hardness of 430-470 HV.
The literature also emphasizes the importance of filler material selection. Using a slightly lower-carbon H13 variant (0.30-0.35% C) as the filler material can reduce cracking tendency without significantly compromising the final hardness after proper heat treatment. Alternatively, a two-layer approach with a low-carbon transition layer followed by standard H13 can provide excellent results with minimal cracking.
Study Insights and Reflections
This study provides valuable quantitative data on the welding current-microstructure-cracking relationship for PTA H13 overlay, filling a gap in the available technical literature. The most significant finding is that the optimal current is not a single value but a function of the entire process strategy, including preheating, post-weld treatment, and the criticality of the component. Engineers must resist the temptation to simply maximize current to reduce cracking, as this can compromise hardness and dimensional accuracy. Instead, a holistic process approach that integrates current selection, thermal management, and post-weld treatment is essential for achieving reliable results in H13 weld overlay applications.