Zhuojin Pipe Fitting Co., LtdMicrostructure and Mechanical Properties of the Transition Layer in Overlay Welding on 35CrMo Steel
Literature Overview
This paper, authored by Xu Yali, Xue Lintao, and Guan Xin from Xinjiang Vocational Institute of Industrial Technology and Baosteel Group Xinjiang Bayi Iron and Steel Co., Ltd., published in the journal Metal Functional Materials in 2022, investigates the microstructure and mechanical properties of the transition layer in a multi-layer overlay welding configuration on 35CrMo low-alloy steel. The study employs the "base metal + transition layer + protective layer" architecture, using ER83-1 low-alloy steel wire for the transition layer and HS112 wear-resistant steel wire for the protective layer, both deposited by GTAW (Gas Tungsten Arc Welding) with argon shielding. This work addresses a fundamental challenge in overlay welding: achieving a metallurgically compatible transition between a medium-carbon low-alloy steel base metal and a high-hardness, high-carbon wear-resistant surface layer.
Welding Process Parameters
The authors selected the following GTAW parameters, which represent a carefully considered process window:
| Parameter | Value | Engineering Rationale |
|---|---|---|
| Welding current | 300 A | Sufficient penetration for base metal melting and transition layer bonding |
| Arc voltage | 25 V | Provides appropriate arc length and heat input |
| Travel speed | 7 mm/s | Controls dilution ratio and solidification rate |
| Shielding gas flow rate | 18 L/min | Adequate protection against atmospheric contamination |
| Welding direction | Left-hand (leftward) | Promotes deeper penetration and better base metal fusion |
| Shielding gas | Argon (Ar) | Inert atmosphere, minimal nitrogen pickup |
The choice of left-hand welding (leftward technique) is particularly noteworthy. In GTAW, the leftward technique directs more heat into the workpiece ahead of the arc, resulting in deeper penetration and a wider, flatter weld bead. For overlay welding applications where good bonding to the base metal is critical, this technique is advantageous because it ensures adequate melting of the base metal surface and promotes a stronger metallurgical bond at the base metal/transition layer interface.
Microstructural Analysis
Base Metal / Transition Layer Interface
The microstructure at the interface between the 35CrMo base metal and the ER83-1 transition layer exhibits a distinctive pattern of alternating thin ferrite layers and pearlite in elongated block morphology. This dispersed distribution pattern is beneficial for improving the plasticity and toughness of the transition layer. The elongated morphology suggests that the solidification was influenced by the thermal gradient direction, with the ferrite and pearlite forming in a lamellar-like arrangement parallel to the heat flow direction.
From a metallurgical perspective, this microstructure is the result of the following factors:
- The carbon content of the ER83-1 wire (approximately 0.08% to 0.12% C) is lower than that of 35CrMo (0.32% to 0.40% C), resulting in a dilution gradient at the interface.
- The chromium and molybdenum content of the ER83-1 wire provides sufficient hardenability to form a balanced ferrite-pearlite microstructure.
- The relatively low cooling rate at the interface (due to the high thermal conductivity of the steel base) allows for the formation of a fine, dispersed microstructure rather than a coarse, brittle one.
Transition Layer / Protective Layer Interface
The microstructure at the transition between the ER83-1 transition layer and the HS112 protective layer consists of fine acicular ferrite, a small amount of blocky ferrite, and pearlite. This microstructure is characteristic of a medium-to-high carbon steel with a controlled cooling rate. The fine acicular ferrite morphology indicates a moderate cooling rate, which is beneficial for toughness.
Mechanical Properties
| Property | Transition Layer | Protective Layer |
|---|---|---|
| Hardness | ~30-35 HRC (estimated) | 47-49 HRC |
| Tensile strength | ~600-650 MPa (estimated) | 765.8 MPa |
| Microstructure | Alternating ferrite and pearlite | Fine acicular ferrite + blocky ferrite + pearlite |
The protective layer achieves a hardness of 47 to 49 HRC, which is well within the target range for wear-resistant applications. The tensile strength of 765.8 MPa is consistent with the high carbon and alloy content of the HS112 wire. The transition layer, while not explicitly quantified for hardness and tensile strength in the abstract, is designed to provide a gradual transition in properties from the base metal to the protective layer, thereby minimizing residual stresses and reducing the risk of cracking.
Engineering Practice Implications
In my extensive experience with overlay welding of medium-carbon and low-alloy steels, the transition layer design is often the most critical aspect of the entire welding configuration. The primary challenge is to bridge the large difference in carbon equivalent (CE) between the base metal and the protective layer. 35CrMo has a CE value of approximately 0.40 to 0.45, while HS112 wear-resistant steel typically has a CE value exceeding 0.70. Without an appropriate transition layer, the high cooling rate at the weld interface can produce hard, brittle martensitic structures susceptible to hydrogen-induced cracking.
The ER83-1 wire selected in this study is an excellent choice for the transition layer because:
- Its carbon content (0.08-0.12%) is significantly lower than both the base metal and the protective layer, reducing the risk of martensite formation.
- Its chromium and molybdenum content provides sufficient hardenability for good strength without excessive brittleness.
- The dilution effect at the base metal/transition layer interface further reduces the effective carbon content, promoting a more ductile microstructure.
I would recommend that engineers considering similar overlay welding configurations on 35CrMo or similar medium-carbon steels pay close attention to the following:
- Preheating the base metal to 150-200°C to reduce cooling rates and minimize hydrogen-induced cracking risk.
- Post-weld stress relief at 550-600°C to relieve residual stresses accumulated during multi-layer deposition.
- Careful control of interpass temperature (150-250°C) to maintain the desired microstructure in each layer.
- Use of low-hydrogen consumables and strict gas protection to minimize hydrogen pickup.
Key Reflections
This paper provides a clear and practical demonstration of how a well-designed transition layer can enable successful overlay welding of dissimilar materials. The microstructural analysis is particularly valuable because it reveals the mechanisms by which the transition layer achieves its metallurgical compatibility function. The alternating ferrite-pearlite structure at the base metal interface is a direct result of the controlled dilution and cooling rate, and its dispersed morphology is a testament to the effectiveness of the selected welding parameters. For engineers working on wear-resistant overlay applications on medium-carbon and low-alloy steels, this study provides a reliable process recipe and a solid understanding of the underlying metallurgy.