Zhuojin Pipe Fitting Co., LtdMicrostructure and Properties of the Transition Layer in 35CrMo Steel Overlay Welding
Literature Overview
The study by Xu Yali, Xue Lintao, and Guan Xin, published in the journal Functional Materials of Metals in 2022, investigates the metallurgical behavior and mechanical performance of a multi-layer overlay welding structure applied to 35CrMo steel. The work is funded by the Xinjiang Autonomous Region Vocational Education Research Project. The authors adopt a three-layer strategy—base metal plus transition layer plus protection layer—to achieve a durable and wear-resistant surface on a medium-carbon alloy steel substrate. This approach is commonly used in applications such as mining equipment, cement mill rollers, and industrial pump shafts where the base material provides structural strength while the surface requires enhanced tribological performance.
Welding Process Parameters and Configuration
The overlay welding was performed using Gas Metal Arc Welding (GMAW) with argon as the shielding gas. The process parameters were carefully selected to balance penetration, dilution, and deposition rate:
| Parameter | Value | Rationale |
|---|---|---|
| Welding current | 300 A | Sufficient for penetration into 35CrMo base metal |
| Welding voltage | 25 V | Maintains stable arc and appropriate arc length |
| Travel speed | 7 mm/s | Controls heat input and bead geometry |
| Gas flow rate | 18 L/min | Ensures complete shielding to prevent atmospheric contamination |
| Welding direction | Left-hand (leftward) | Promotes better penetration and bead shape |
| Transition layer wire | ER83-1 (low-alloy steel) | Provides ductility and reduces thermal stress |
| Protection layer wire | HS112 (wear-resistant steel) | Delivers high hardness and abrasion resistance |
The left-hand welding direction is a notable choice. In GMAW, left-hand welding typically results in a wider, flatter bead with deeper penetration compared to right-hand welding. This is advantageous for the transition layer, where sufficient bonding with the base metal is critical.
Microstructural Analysis
The microstructure of the transition layer (between base metal and transition layer interface) shows alternating bands of thin ferrite and pearlite in a long block-like morphology. This dispersed distribution pattern is beneficial because it provides a gradient in mechanical properties, reducing the risk of brittle fracture at the interface. The presence of thin ferrite films between pearlite colonies enhances ductility and toughness, which is essential for accommodating residual stresses from the welding thermal cycle.
The interface between the transition layer and the protection layer exhibits fine acicular ferrite, a small amount of blocky ferrite, and pearlite. Acicular ferrite is a desirable microstructural constituent in weld metals because it provides an excellent combination of strength and toughness. The fine grain size of the acicular ferrite in this region suggests that the cooling rate was sufficient to suppress the formation of coarse grain boundary carbides and retained austenite.
Mechanical Performance
The protection layer achieved a hardness range of 47 to 49 HRC, which corresponds to a wear-resistant grade suitable for moderate abrasive conditions. The tensile strength of the overlay structure reached 765.8 MPa, indicating good load-bearing capacity. These values suggest that the multi-layer approach successfully combines the toughness of the base metal with the hardness of the wear-resistant overlay, without creating a brittle interlayer that would compromise structural integrity.
The transition layer serves a dual purpose: it acts as a metallurgical buffer to accommodate the large difference in thermal expansion coefficients between the base metal and the protection layer, and it provides a ductile zone that can absorb plastic deformation during service loading. The alternating ferrite-pearlite morphology observed at the base metal-transition layer interface is consistent with a controlled solidification pattern that promotes crack resistance.
Engineering Practice Considerations
In practical applications, the dilution ratio between the base metal and the transition layer is a critical variable. If dilution is too high, the transition layer may inherit excessive hardness from the 35CrMo base, reducing its ductility function. If dilution is too low, the bonding strength may be insufficient. The selected parameters—300 A, 25 V, 7 mm/s—appear to achieve an optimal dilution balance, as evidenced by the sound microstructure and mechanical properties reported.
For engineers applying this technique to similar substrates, the following considerations are relevant:
- Preheating may be necessary for thicker sections of 35CrMo steel to reduce the cooling rate and prevent martensite formation in the heat-affected zone.
- Post-weld heat treatment (PWHT) should be evaluated to relieve residual stresses, particularly if the component is subjected to cyclic loading.
- The weld sequence should be planned to minimize distortion, using techniques such as staggered welding or back-step welding for long overlay areas.
- Dilution testing on coupon samples prior to production welding is recommended to verify the transition layer composition.
Study Insights and Outlook
This paper demonstrates that a carefully designed multi-layer overlay strategy can significantly enhance the surface performance of medium-carbon alloy steels without compromising the structural integrity of the base metal. The use of ER83-1 as a transition layer material is a practical and cost-effective choice, as it provides adequate ductility and weldability with 35CrMo. The reported hardness and tensile strength values are competitive with other wear-resistant overlay systems, making this approach suitable for industrial applications where moderate wear resistance is required.
A limitation of this study is the absence of fatigue testing and impact testing data. In applications involving cyclic loading, such as rotating equipment, the fatigue life of the overlay structure is a critical parameter. Future research should investigate the long-term durability of this overlay system under combined wear and fatigue conditions, as well as the effect of different cooling rates on the microstructure and properties of the transition layer.