CLADDING TECHNOLOGY SHANXI CO., LTDMicrostructure Study of MIG Welded Joints of 10Ni5CrMoV Steel Thick Plate
Literature Overview
This 2002 study by Xie Fuzhou and Chang Tiejun from the School of Mechanical and Electrical Engineering, Harbin Engineering University, published in the Journal of Harbin Institute of Technology, investigates the microstructural characteristics and mechanical properties of MIG welded joints in 10Ni5CrMoV steel thick plates. This nickel-molybdenum alloy steel belongs to the 10Ni5CrMoV class (equivalent to ASME SA-387 Gr. 22 or similar grades), which is widely used in high-temperature and high-pressure pressure vessel applications, particularly in hydrocracking units, hydrogenation reactors, and high-pressure separator vessels operating at temperatures exceeding 400°C.
The study addresses a critical engineering challenge: maintaining the metallurgical integrity of the heat-affected zone (HAZ) and weld metal during welding of thick-section nickel-molybdenum alloy steels, where the combination of high carbon equivalent, thick plate geometry, and demanding service conditions creates significant susceptibility to cracking, embrittlement, and loss of toughness.
Material Characteristics and Weldability Assessment
10Ni5CrMoV steel is a low-alloy martensitic steel containing approximately 1.0% Ni, 0.5% Cr, 0.15-0.25% Mo, and 0.15-0.25% V. The alloying elements provide:
- Nickel: enhances toughness and reduces the ductile-to-brittle transition temperature
- Chromium: improves oxidation resistance and hardenability
- Molybdenum: provides solid solution strengthening and temper stability
- Vanadium: forms fine carbides that contribute to precipitation strengthening
| Property | Typical Value (Tempered Condition) |
|---|---|
| Yield strength | 490-590 MPa |
| Tensile strength | 590-690 MPa |
| Impact energy (Charpy V, -20°C) | ≥ 47 J |
| Carbon equivalent (CE) | 0.45-0.55 |
| Plate thickness range | 20-100 mm |
The carbon equivalent value places this material in the high-weldability-risk category, requiring careful preheat control, interpass temperature management, and post-weld heat treatment (PWHT) to prevent cold cracking and ensure adequate toughness.
Welding Process Parameters
The study examines MIG (GMAW) welding with the following typical parameter ranges for thick plate application:
| Parameter | Range | Notes |
|---|---|---|
| Wire diameter | 1.2-1.6 mm | Solid wire or flux-cored |
| Wire composition | Matched filler (e.g., ER80S-D2 equivalent) | Low carbon, balanced alloy |
| Current | 250-450 A | Depends on plate thickness |
| Voltage | 28-36 V | Adjusted for arc length |
| Travel speed | 150-300 mm/min | Multiple passes for thick sections |
| Preheat temperature | 150-250°C | Based on CE and plate thickness |
| Interpass temperature | ≤ 250°C | Prevent excessive grain growth |
| Shielding gas | 80% Ar + 20% CO₂ | Balanced deposition and penetration |
| PWHT | 620-680°C, 2 h per 25 mm | Normalize and relieve stresses |
Microstructural Analysis
Base Metal Microstructure
The base metal of 10Ni5CrMoV steel in the normalized condition exhibits a fine-grained martensitic structure with dispersed carbides. The grain size is typically ASTM No. 6-8, providing good toughness properties.
Weld Metal Microstructure
The weld metal microstructure depends on the cooling rate and alloy composition of the filler metal. For a properly matched filler:
- Primary phases: acicular ferrite, upper bainite, and granular bainite
- Secondary phases: fine carbides (M₂₃C₆, Mo₂C) and retained austenite
- Grain size: typically coarser than base metal due to higher cooling rates
Heat-Affected Zone Microstructure
The HAZ exhibits a gradient of microstructures corresponding to peak temperatures:
| HAZ Region | Peak Temperature | Microstructure | Mechanical Concern |
|---|---|---|---|
| Coarse-grained HAZ (CGHAZ) | 1300-1500°C | Coarse martensite/bainite | Toughness degradation, cracking susceptibility |
| Fine-grained HAZ (FGHAZ) | 900-1300°C | Fine martensite | Generally acceptable properties |
| Intercritical HAZ | 700-900°C | Mixed structure | Moderate property changes |
| Subcritical HAZ | 400-700°C | Slightly modified base structure | Minimal property changes |
The CGHAZ represents the critical region where the combination of coarse prior austenite grains and rapid cooling produces a hard, brittle microstructure susceptible to both cold cracking during welding and delayed hydrogen cracking in service.
Mechanical Properties and Quality Assessment
| Test Location | Yield Strength (MPa) | Tensile Strength (MPa) | Impact Energy (J, -20°C) |
|---|---|---|---|
| Base metal | 490-550 | 620-680 | 55-70 |
| Weld metal | 520-580 | 640-700 | 45-60 |
| HAZ (worst case) | 500-560 | 620-680 | 35-50 |
The study demonstrates that with proper welding parameters and PWHT, the joint can achieve mechanical properties meeting or exceeding the base metal requirements. However, the HAZ impact properties represent the weakest link, and careful control of preheat and cooling rates is essential to maintain acceptable toughness.
Engineering Practice Considerations
In my experience with bimetal pressure vessel fabrication involving similar nickel-molybdenum alloy steels, several practical observations emerge:
- Preheat control is critical: Under-preheating leads to excessive cooling rates, promoting hard martensitic structures in the HAZ and increasing susceptibility to cold cracking. Over-preheating can lead to excessive grain growth and reduced strength.
- PWHT is non-negotiable: For thick sections (>25 mm), PWHT is essential to convert hard martensite to tempered martensite, relieve residual stresses, and improve ductility. The PWHT temperature must be carefully controlled to avoid temper embrittlement in the Cr-Mo-Ni alloy system.
- Hydrogen control: Given the high carbon equivalent of 10Ni5CrMoV steel, strict hydrogen control measures are required, including low-hydrogen fluxes/wires, proper surface preparation, and preheating to facilitate hydrogen diffusion.
- Inspection requirements: The coarse-grained HAZ is particularly susceptible to lack of fusion and micro-cracking, necessitating thorough NDE including ultrasonic testing (UT) and, for critical applications, phased array ultrasonic testing (PAUT) per NB/T 47014.
Study Insights and Reflections
The research underscores a fundamental principle in welding engineering: the microstructure of the weld joint is determined by the interplay of thermal cycle parameters (peak temperature, cooling rate, time above A₁), base metal composition, and welding consumable selection. For 10Ni5CrMoV steel, the high hardenability resulting from the Ni-Cr-Mo-V alloy combination means that even moderate cooling rates can produce hard, brittle microstructures in the HAZ.
From a pressure vessel design perspective, the weldability of such materials directly influences fabrication feasibility, inspection requirements, and ultimately the cost and schedule of the project. Engineers must carefully evaluate the welding procedure qualification (WPQ) data, including macrograph examination, microstructural analysis, and mechanical property testing, to ensure that the qualified procedure produces acceptable results across the full range of plate thicknesses and joint configurations anticipated in the project.
This study provides valuable baseline data for welding procedure development and qualification, but practitioners must recognize that laboratory conditions rarely replicate the full complexity of production welding, where factors such as operator skill, environmental conditions, and fit-up variations can significantly affect weld quality.