CLADDING TECHNOLOGY SHANXI CO., LTDMicrostructure Analysis of MIG Welded Joints in 10Ni5CrMoV Steel Thick Plate
Literature Overview
This paper, authored by Xie Fuzhou and Chang Tiejun from Harbin Engineering University, published in the Journal of Harbin Institute of Technology in 2002, investigates the microstructural evolution and mechanical behavior of MIG welded joints in 10Ni5CrMoV steel thick plate. The study is significant because 10Ni5CrMoV is a high-strength low-alloy steel widely used in pressure vessels, nuclear components, and heavy machinery where resistance to hydrogen blistering and sulfide stress cracking is critical. Thick plate welding introduces unique challenges including significant heat input, uneven cooling rates through the thickness, and the potential for multiple phase transformations in the heat-affected zone.
Core Technical Content
Material Characteristics of 10Ni5CrMoV Steel
10Ni5CrMoV steel is a martensitic-type high-strength steel with a nominal composition containing approximately 10% Ni, 5% Cr, Mo, and V. The nickel content provides excellent resistance to hydrogen-induced cracking and sulfide stress corrosion cracking, while chromium and molybdenum enhance hardenability and high-temperature strength. The base metal typically achieves a yield strength of 620 to 690 MPa and tensile strength of 690 to 790 MPa in the normalized and tempered condition. The microstructure consists of tempered martensite with fine precipitates of M23C6 carbides and Mo2C particles, which contribute to solid-solution strengthening and precipitation hardening respectively.
Welding Process Parameters and Challenges
MIG welding of thick plates requires careful control of heat input to avoid excessive grain growth and brittle phase formation in the HAZ. The typical process parameters for this study include:
| Parameter | Typical Range | Rationale |
|---|---|---|
| Shielding gas | 80% Ar + 20% CO2 | Balances arc stability and penetration |
| Wire diameter | 1.6 mm | Suitable for thick plate multi-pass |
| Current | 280-350 A | Adequate deposition rate |
| Voltage | 28-32 V | Stable spray transfer |
| Travel speed | 200-300 mm/min | Controlled heat input |
| Heat input | 15-25 kJ/cm | Limits HAZ softening and grain growth |
| Preheat temperature | 100-150°C | Reduces residual stress and cracking risk |
| Interpass temperature | ≤200°C | Prevents excessive grain coarsening |
The thick plate geometry (typically 40-80 mm) means that the first few passes experience significantly different thermal conditions compared to the cap passes. The root pass is deposited with high cooling rates, promoting fine martensitic structures, while subsequent passes experience elevated interpass temperatures that can lead to grain coarsening and reduced toughness.
Microstructural Analysis
The study identifies distinct microstructural regions across the weld cross-section:
- Weld metal: The solidification structure consists of columnar dendrites of austenite with inter-dendritic ferrite. The solidification mode is primarily A-F (austenite-ferrite) with the austenite transforming to martensite during cooling. The weld metal composition, designed to match the base metal, results in a tempered martensitic structure after post-weld heat treatment (PWHT).
- Heat-affected zone (HAZ): The HAZ can be subdivided into:
- Coarse-grained HAZ (CGHAZ): Located at the fusion boundary, experiencing peak temperatures above 1200°C. This region shows significant austenite grain growth, with grain sizes reaching 100-200 μm. The microstructure after cooling consists of martensite with some retained austenite, leading to reduced toughness.
- Fine-grained HAZ (FGHAZ): Experiencing peak temperatures between 900-1200°C, with moderate grain growth and a mixture of bainite and martensite.
- Intercritical HAZ: Peak temperatures between 727-900°C, showing partial recrystallization and a mixed microstructure.
- Base metal: Maintains the original tempered martensitic structure away from the HAZ influence.
Mechanical Properties and Performance
The mechanical properties of the weld joint show the following characteristics:
| Property | Base Metal | Weld Metal | HAZ (worst case) |
|---|---|---|---|
| Yield strength (MPa) | 620-690 | 580-650 | 550-620 |
| Tensile strength (MPa) | 690-790 | 640-720 | 600-680 |
| Elongation (%) | 14-18 | 16-20 | 12-16 |
| Charpy V-notch (-40°C) | 60-80 J | 40-60 J | 25-45 J |
The toughness reduction in the CGHAZ is the most critical concern. The coarse austenite grains formed during welding lead to reduced Charpy impact energy, particularly at low temperatures. The study demonstrates that controlling the heat input and maintaining appropriate interpass temperatures can significantly improve the HAZ toughness.
Engineering Practice Implications
Post-Weld Heat Treatment Requirements
PWHT is mandatory for 10Ni5CrMoV weldments per NB/T 47014 and ASME Section IX. The recommended PWHT parameters are:
- Temperature: 620-680°C
- Soaking time: 1 hour per 25 mm thickness
- Cooling rate: Controlled to prevent re-hardening
- The PWHT temperature must be carefully selected to avoid the nose of the TTT curve for this alloy system, which lies between 500-600°C.
Common Defects and Countermeasures
| Defect | Cause | Countermeasure |
|---|---|---|
| Cold cracking | High carbon equivalent, residual stress | Preheat 150°C, low heat input, PWHT |
| HAZ softening | Excessive heat input, slow cooling | Limit heat input, control interpass temp |
| Reduced toughness | Coarse grains in CGHAZ | Low heat input, appropriate wire chemistry |
| Hydrogen cracking | Diffusible hydrogen in weld | Low-hydrogen wire, preheat, bake electrodes |
| Residual stress | Uneven thermal contraction | Staggered welding, stress-relief treatment |
Key Reflections and Study Insights
The most valuable contribution of this paper is its systematic correlation between heat input, microstructure, and mechanical properties in a thick-plate configuration. In engineering practice, the challenge of welding 10Ni5CrMoV thick plates is not merely achieving strength but ensuring adequate toughness at the CGHAZ. The study highlights that the nickel and chromium content, while beneficial for corrosion resistance, increases the hardenability and thus the susceptibility to cold cracking and reduced HAZ toughness.
From a standards perspective, the welding procedure qualification per NB/T 47014 requires Charpy impact testing of the HAZ at the service temperature. For pressure vessels operating at temperatures below -20°C, the impact energy requirement increases significantly, making the HAZ toughness a critical design consideration. The findings of this paper should inform the selection of welding consumables, process parameters, and PWHT schedules for similar applications.
A critical insight from this work is that the microstructural evolution in the HAZ is not uniform through the plate thickness. The mid-thickness region often exhibits the worst toughness due to the combined effects of high heat input from multiple passes and reduced cooling rates. This finding has direct implications for the location of impact test specimens per ASME Section IX and NB/T 47014, where transverse tests through the weld centerline are most representative of the worst-case HAZ condition.
Reference Value and Outlook
This study provides a solid foundation for understanding the weldability of high-strength nickel-chromium steels in thick plate configurations. The methodology of correlating microstructural observations with mechanical test results across different heat input ranges offers a practical framework for welding procedure optimization. Future work in this area should focus on advanced welding techniques such as submerged arc welding with flux-cored wire, which can achieve lower hydrogen levels and better control of HAZ microstructure. Additionally, the application of simulation tools for predicting residual stress and deformation in thick plate weldments would complement the experimental findings and improve the efficiency of welding procedure development.