How can Q275 steel weld quality be assessed based on interpass temperature during welding?

Interpass temperature is a critical process parameter in multi-pass, multi-pass welding of Q275 steel (referring to the residual temperature of the previous weld pass before the subsequent weld pass). Its value directly affects the microstructure, stress state, and defect propensity of the weld and heat-affected zone (HAZ). By monitoring the deviation of the interpass temperature from the appropriate range, a preliminary assessment can be made regarding weld quality risks. The specific judgment logic and basis are as follows:
1. Defining the appropriate interpass temperature range for Q275 steel
Q275 is a low-carbon, killed steel (carbon content approximately 0.18%-0.28%) with slightly higher strength than Q235 and moderate hardening tendency. The interpass temperature should be adjusted based on plate thickness, welding method, and ambient temperature. The key objective is to avoid quench cracking caused by rapid cooling and to prevent grain coarsening caused by overheating. For thin plates (thickness ≤ 10mm): the interpass temperature is recommended to be controlled between 150-250°C.
For thick plates (thickness > 10mm) or highly rigid structures: the interpass temperature is recommended to be controlled between 200-300°C (preheating is required to prevent excessive cooling in a single pass).
When welding in a low-temperature environment (<0°C): the interpass temperature should be increased to 250-350°C (to compensate for excessive heat dissipation from the environment).
II. Quality Assessment When the Interpass Temperature is Too Low (Below the Lower Limit of the Reasonable Range)
Excessively low interpass temperatures indicate that the previous weld pass cooled too quickly, resulting in insufficient "tempering" of the heat-affected zone and weld metal. This can easily lead to the following quality issues, which can be identified by combining temperature records with defect characteristics:
1. Increased Risk of Cold Cracks
Principle: At low temperatures, the heat-affected zone of the previous weld pass tends to form a hardened structure (such as martensite). Furthermore, hydrogen in the weld (from the electrode coating or base metal oil) is difficult to diffuse out, resulting in hydrogen-induced cracks at stress concentration points. Judgment Basis:
Temperature records show that the interpass temperature is consistently below 150°C (for thin plate) or 200°C (for thick plate);
Fine transverse or longitudinal cracks may appear on the weld surface or near the weld (mostly cold cracks, which may develop within hours after welding);
Nondestructive testing (such as UT and MT) may reveal linear defects near the heat-affected zone.
2. Decreased Toughness of the Weld and Heat-Affected Zone
Principle: Rapid cooling results in insufficient microstructure refinement in the weld metal and heat-affected zone, coarsening the pearlite lamellae, and impact toughness (e.g., AKV) may fall below the design requirement (typically ≥27J for Q275 welds).
Judgment Basis:
The interpass temperature is consistently below the acceptable range, and post-weld tempering is not performed;
Mechanical property testing shows a significant decrease in impact energy (e.g., <20J), indicating brittle fracture. III. Quality Assessment When the Interpass Temperature is Excessively High (Exceeding the Upper Limit of the Reasonable Range)
Excessively high interpass temperatures can cause the weld area to remain at elevated temperatures for extended periods, leading to overheating. Quality defects can be identified by the following characteristics:
1. Increased Risk of Thermal Cracking (Crystallization Cracking)
Principle: At high temperatures, the weld metal grains grow excessively, and low-melting-point impurities (such as sulfur and phosphorus) segregate at the grain boundaries, forming a "liquid film." Under the action of welding stress, cracks develop along the grain boundaries (thermal cracks often occur in the weld center or near the fusion line).
Judgment Basis:
Temperature records indicate that the interpass temperature exceeds 300°C (for thick plate) or 350°C (for low-temperature environments);
Longitudinal cracks may appear along the length of the weld surface (thermal cracks are often visible on the surface and appear immediately after welding);
Macroscopic inspection reveals signs of "overburned" weld metal (such as severe surface oxidation and a bluish color). 2. Deterioration of the strength and toughness of the weld and heat-affected zone
Principle: Overheating causes coarsening of the austenite grains in the weld metal and heat-affected zone (the grain size may drop from grade 8 to below grade 4). Upon cooling, coarse pearlite and ferrite form, resulting in a 5%-10% decrease in strength (such as tensile strength) and a significant reduction in toughness.
Judgment basis:
Sustained excessive interpass temperature (e.g., >300°C) and high weld heat input (high current, slow speed);
Mechanical property tests show tensile strength below the parent material standard (Q275 standard tensile strength 410-540 MPa), and the impact energy may drop below 20 J;
Metallographic analysis reveals coarse grains in the weld (>50 μm) and the presence of a "coarse-grained zone" in the heat-affected zone.
3. Increased welding distortion
Principle: High temperatures increase the plasticity of the material. Excessively high interpass temperatures lead to excessive thermal expansion in the weld area, resulting in uneven contraction after cooling, leading to greater angular or bending deformation. Judgment Basis:
The interpass temperature exceeds the acceptable range and the structural rigidity is low (e.g., thin plate splicing);
Post-weld geometric dimensional deviation exceeds the standard (e.g., angular distortion > 3°/m), requiring correction to meet the standard.
IV. Quality Judgment When the Interpass Temperature is Within the Acceptable Range
If the interpass temperature is stably controlled within the recommended range (150-350°C, adjusted based on plate thickness and ambient conditions), this generally indicates that during welding:
The cooling rate is moderate, the heat-affected zone (HAZ) hardening is low (primarily pearlite + ferrite, with no significant martensite), and the risk of cold cracking is low;
The weld metal grains are not excessively grown, impurity segregation is not significant, and the risk of hot cracking is low;
Hydrogen diffusion is sufficient (when the interpass temperature is ≥150°C, the hydrogen diffusion coefficient increases significantly), and the likelihood of delayed cracking is low.
At this point, the weld quality can be preliminarily judged to be acceptable, but final verification still requires combined visual inspection (no porosity or lack of fusion) and nondestructive testing.