Weld Overlay Characteristics of Tempered 42Cr2Mo Steel

Literature Overview

This 2010 publication from Nanchong Vocational and Technical College examines the weldability and overlay characteristics of 42Cr2Mo steel in the quenched and tempered condition. The 42Cr2Mo alloy steel is widely used in high-strength shafts, crankshafts, and structural components in heavy machinery and automotive applications. The tempered condition represents a critical metallurgical state where the balance between strength and toughness is optimized, making it particularly challenging for subsequent welding operations.

Metallurgical Background and Welding Challenges

The 42Cr2Mo steel in the tempered condition typically exhibits a hardness of 28–35 HRC, with a microstructure consisting of tempered martensite with fine carbide precipitates. The alloy contains approximately 0.40–0.50% C, 1.80–2.20% Cr, 0.15–0.25% Mo, and 0.90–1.20% Mn, which collectively contribute to excellent hardenability but also increase the susceptibility to cold cracking during welding.

Base Metal Properties in Tempered Condition

Property	Typical Value	Welding Implication
Hardness	28–35 HRC	Above critical weldability threshold (25 HRC)
Carbon equivalent (CE)	0.45–0.55	High cracking susceptibility
Tensile strength	900–1100 MPa	High residual stress potential
Impact energy (Charpy V)	40–60 J at 20°C	Moderate toughness
Yield strength	700–850 MPa	High constraint on weld metal ductility

Overlay Process Parameters and Microstructural Evolution

The study investigates multiple overlay processes including submerged arc welding (SAW), gas metal arc welding (GMAW), and flux-cored arc welding (FCAW) applied to the tempered 42Cr2Mo substrate. The primary challenge is controlling the dilution between the overlay material and the high-alloy base metal to prevent excessive hardening and cracking in the weld metal and heat-affected zone.

Welding Process	Current (A)	Voltage (V)	Travel Speed (mm/min)	Dilution (%)	Overlay Hardness (HRC)
SAW (single pass)	400–500	28–34	200–300	15–25	35–42
SAW (multi-pass)	350–450	26–32	250–350	8–15	30–38
GMAW	180–250	20–26	200–300	10–20	32–40
FCAW	250–350	28–34	200–280	12–18	33–41

Microstructural Zones in the Overlay/Weld Interface

The weld overlay on tempered 42Cr2Mo steel creates several distinct microstructural zones that must be controlled:

Overlay weld metal: Composition depends on dilution; may contain martensite, bainite, or mixed microstructures depending on cooling rate and alloy content
Dilution zone: Transition region where base metal composition gradually changes; often exhibits the highest hardness due to local carbon enrichment
Heat-affected zone (HAZ): Subject to re-austenitization and rapid cooling; may develop hard martensitic structures exceeding 45 HRC
Base metal: Remains in tempered condition beyond the HAZ boundary

Cracking Susceptibility and Prevention Strategies

The tempered condition of 42Cr2Mo steel presents a significant cold cracking risk due to the combination of high carbon equivalent, retained hardness, and hydrogen pickup from the welding process. The study identifies several critical control measures:

Control Measure	Specification	Effectiveness
Pre-heat temperature	250–350 °C	Reduces cooling rate below critical
Interpass temperature	250–350 °C	Maintains thermal balance
Post-weld heat treatment	600–650 °C for 2 h per 25 mm thickness	Stress relief and hydrogen diffusion
Consumable selection	Low-hydrogen electrodes (E71T-8, E81T-Ni2)	Minimizes hydrogen pickup
Hydrogen control	Bake electrodes at 350–400 °C for 1–2 h	Reduce diffusible hydrogen to <5 ml/100g
Weld design	Avoid hard stops; use run-out plates	Reduce stress concentration

Study Insights and Engineering Practice

The research confirms that overlay welding on tempered 42Cr2Mo steel is feasible but requires stringent process control. The multi-pass approach with reduced dilution is strongly preferred over single-pass operations, as it allows for better control of the thermal cycle and reduces the risk of cracking. From a standards perspective, the weld procedure must be qualified per NB/T 47014 or ASME IX, with specific attention to the pre-heat and post-weld heat treatment requirements. The dilution analysis is particularly important for engineers designing overlay systems on high-strength alloy steels, as even modest dilution (10–15%) can significantly alter the weld metal microstructure and mechanical properties. The practical implication is that for repair or cladding operations on in-service 42Cr2Mo components, the tempering condition must be documented, and the welding procedure must be specifically qualified for that condition rather than relying on procedures developed for the normalized or annealed condition.