Effect of Preheating and Post-Weld Tempering on Bimetallic Gradient Hardfacing of Forging Dies on Cast Steel Base

Technical Context and Objective

Forging dies are critical tooling components in metal forming operations, subjected to extreme mechanical loads, thermal cycling, and abrasive wear. To extend die life, bimetallic gradient hardfacing is employed to create a transition from a tough base material to a hard, wear-resistant surface layer. The study investigates the influence of preheating temperature and post-weld tempering on the microstructure and mechanical properties of the hardfacing layer applied to cast steel forging dies. This topic is particularly relevant for engineers involved in die maintenance and repair, where the selection of appropriate thermal treatment parameters directly impacts die service life and manufacturing costs.

Experimental Setup and Parameters

The hardfacing was performed on a high-manganese cast steel forging die (ZGMn13 equivalent) using multi-layer hardfacing with a gradient composition design. The hardfacing wire contained chromium carbide (Cr₇C₃) particles in a high-chromium cast iron matrix.

Parameter	Condition A	Condition B	Condition C	Condition D
Preheat temperature	100°C	200°C	300°C	400°C
Post-weld tempering	None	550°C / 2h	600°C / 2h	650°C / 2h
Welding method	SAW	SAW	SAW	SAW
Welding current	400 A	400 A	400 A	400 A
Travel speed	200 mm/min	200 mm/min	200 mm/min	200 mm/min
Number of layers	3	3	3	3
Total layer thickness	~4 mm	~4 mm	~4 mm	~4 mm

Microstructural Evolution

Effect of Preheating Temperature

The preheating temperature had a profound effect on the microstructure of both the hardfacing layer and the heat-affected zone (HAZ):

Preheat Temp	HAZ Hardness (HV)	HAZ Microstructure	Interface Cracks	Dilution Zone Width
100°C	580-620	Fully martensitic, brittle	Multiple microcracks	1.2-1.5 mm
200°C	480-520	Martensite + some bainite	Few microcracks	1.0-1.2 mm
300°C	380-420	Bainite + tempered martensite	No cracks	0.8-1.0 mm
400°C	320-360	Bainite + pearlite	No cracks	0.6-0.8 mm

At low preheat temperatures (100°C), the rapid cooling rate resulted in complete martensitic transformation in the HAZ, producing excessive hardness and brittleness. The high residual stresses in this condition led to the formation of microcracks at the interface, which could serve as crack initiation sites during service. As the preheat temperature increased, the cooling rate decreased, promoting bainitic transformation and reducing HAZ hardness. At 300°C and above, the HAZ microstructure consisted primarily of bainite with some tempered martensite, providing adequate toughness without significant cracking.

Effect of Post-Weld Tempering

Post-weld tempering at different temperatures produced distinct microstructural changes in the hardfacing layer:

Tempering Temp	Hardfacing Hardness (HV)	Carbide Morphology	Retained Austenite	Residual Stress Level
None (as-deposited)	1100-1180	Fine, dispersed Cr₇C₃	15-20%	High (compressive ~300 MPa)
550°C / 2h	950-1020	Slightly coarsened Cr₇C₃	10-15%	Moderate (compressive ~150 MPa)
600°C / 2h	880-950	Coarsened Cr₇C₃, some aggregation	5-10%	Low (compressive ~80 MPa)
650°C / 2h	820-880	Significant coarsening, primary carbide formation	<5%	Very low (near zero)

The tempering process progressively reduced the hardness of the hardfacing layer while improving toughness and reducing residual stresses. At 550°C, the tempering produced a favorable balance between hardness and toughness, with minimal coarsening of the Cr₇C₃ carbides. At higher temperatures (650°C), excessive carbide coarsening and aggregation reduced the hardness more significantly, potentially compromising wear resistance.

Mechanical Property Results

Hardness Profile Across the Cross-Section

The hardness gradient from the hardfacing surface to the base metal was characterized for each condition:

Condition	Surface (HV)	Mid-layer (HV)	Interface (HV)	HAZ (HV)	Base (HV)
A (100°C, no temper)	1150	1100	950	600	250
B (200°C, 550°C temper)	980	950	800	500	250
C (300°C, 600°C temper)	920	880	750	400	250
D (400°C, 650°C temper)	850	820	700	340	250

Condition C (300°C preheat, 600°C tempering) produced the most favorable hardness gradient, providing high surface hardness with a smooth transition to the base metal, minimizing stress concentration at the interface.

Impact Toughness

Charpy V-notch impact tests were conducted on specimens from the hardfacing layer:

Condition	Impact Energy (J)	Failure Mode
A (100°C, no temper)	8-12	Brittle cleavage
B (200°C, 550°C temper)	18-22	Mixed ductile-brittle
C (300°C, 600°C temper)	25-30	Predominantly ductile
D (400°C, 650°C temper)	28-35	Ductile

The improvement in impact toughness with increasing preheat and tempering temperatures is attributed to the reduction of brittle martensite and the relief of residual stresses. Condition C provided the best balance of toughness and hardness for forging die applications.

Wear Resistance Testing

Dry sliding wear tests against steel counterfaces (HRC 60) were performed:

Condition	Wear Rate (mg/N·m)	Relative Wear Resistance
Base metal (unhardfaced)	8.5	1.0×
A (100°C, no temper)	2.1	4.0×
B (200°C, 550°C temper)	1.8	4.7×
C (300°C, 600°C temper)	1.6	5.3×
D (400°C, 650°C temper)	1.9	4.5×

Condition C exhibited the best wear resistance, demonstrating that the optimal tempering temperature maximizes the synergistic effect of hard carbides and a tough matrix.

Recommended Process Window

Based on the comprehensive analysis, the recommended process parameters for bimetallic gradient hardfacing of forging dies on cast steel bases are:

Parameter	Recommended Range	Rationale
Preheat temperature	250-350°C	Prevent HAZ cracking, control cooling rate
Interpass temperature	200-300°C	Maintain thermal balance
Post-weld tempering	550-600°C for 2 hours	Optimal hardness-toughness balance
Layer thickness per pass	1.0-1.5 mm	Control dilution and dilution gradient
Total layer thickness	3-5 mm	Adequate wear protection
Post-weld cooling	Controlled cooling (≤5°C/min)	Prevent thermal cracking

Engineering Practice Considerations

In practical forging die applications, the selection of process parameters must consider the specific service conditions. For hot forging operations where thermal cycling is severe, a slightly higher tempering temperature (600-650°C) may be beneficial to improve thermal fatigue resistance, accepting a modest reduction in hardness. For cold forging applications where wear resistance is the primary concern, lower tempering temperatures (550°C) can be employed to maintain higher hardness.

The study also highlights the importance of the dilution gradient in bimetallic hardfacing. A smooth transition in hardness and composition from the hardfacing layer to the base metal minimizes stress concentration and improves the overall fatigue resistance of the die. The recommended multi-layer approach with gradually changing composition achieves this gradient effectively.

Conclusion

The study demonstrates that preheating and post-weld tempering are critical process parameters for bimetallic gradient hardfacing of forging dies on cast steel bases. The optimal combination of 300°C preheat temperature and 600°C tempering for 2 hours produces the best balance of surface hardness (920-950 HV), impact toughness (25-30 J), and wear resistance (5.3× improvement over base metal). The smooth hardness gradient achieved under these conditions minimizes stress concentration at the interface, enhancing the fatigue life of the die. Engineers involved in forging die maintenance should adopt these recommended process parameters to maximize die service life while maintaining cost-effective manufacturing practices. The findings provide a solid foundation for process optimization in similar bimetallic hardfacing applications.