Destructive welding test

Destructive welding test is a core tool for welding quality control and process evaluation.Destructive welding test refers to a series of experimental methods that involve applying loads to weld joints through physical or mechanical methods until they fail, then inspecting and testing their performance to evaluate weld quality, process rationality, and material suitability.

一.Destructive welding test：Core Purpose and Significance

The primary purpose of destructive testing is not to inspect every product, but rather to:

① Process Qualification: Before mass production begins, verify whether the selected welding method, materials, and parameters can produce joints that meet design requirements (such as strength, toughness, and corrosion resistance).

② Welder Qualification Certification: Assess the technical level of welders or welding operators to ensure they are capable of producing acceptable welds.

③ Quality Monitoring and Random Inspection: During mass production, samples are regularly sampled from the production line for destructive testing to monitor the consistency of welding quality.

④ Failure Analysis: When a welded structure fails, destructive testing is performed to analyze the fracture surface and identify the root cause of the failure (such as incomplete penetration, cracks, slag inclusions, etc.).

⑤ New Material/Process R&D: When developing new welding materials or processes, evaluate their performance limits.

二. Common Destructive Welding Test Methods

There are many types of destructive tests, which can be categorized as follows based on the properties being evaluated:

① Mechanical Properties Tests

These tests primarily evaluate the load-bearing capacity and deformation capacity of welded joints.

● Tensile Test:
▶ Purpose: Determines the tensile strength, yield strength, elongation after fracture, and reduction of area of ​​welded joints.

▶ Method: A standard specimen is broken on a tensile testing machine. The specimen can be cut from the weld metal or a full weld tensile specimen that includes the weld, heat-affected zone, and base metal.

▶ Problems Detected: Insufficient weld strength and poor ductility.

● Bend Test:
▶ Purpose: Evaluates the ductility and surface quality of welded joints. Inspects the weld for defects such as cracks and lack of fusion.

▶Method: Bend the specimen around a bend of a certain diameter to a specified angle (e.g., 180°) and observe whether cracks form on the tensile surfaces (front bend, back bend, and side bend).

▶Problems that may be detected: Poor weld plasticity, internal or surface defects.

●Impact Test (Charpy V-Notch Impact Test):
▶Purpose: Determine the toughness of a weld joint (usually the weld center and heat-affected zone) under impact loading, that is, its resistance to brittle fracture.

▶Method: Place a notched standard specimen on an impact testing machine, fracture it with a pendulum, and measure the absorbed impact energy.

▶Problems that may be detected: Insufficient toughness of the weld joint (especially the heat-affected zone) and prone to brittle fracture.

●Hardness Test:
▶Purpose: Measure the hardness distribution of different regions of a weld joint (base material, heat-affected zone, weld) to assess the material’s hardening tendency and microstructural changes.

▶Method: Use a Vickers, Brinell, or Rockwell hardness tester to measure points at regular intervals across the cross-section of the joint.

▶Problems that can be detected: Excessive hardness in the heat-affected zone (which may lead to cold cracks) and uneven hardness.

② Macro and micro metallographic examinations

This type of test assesses the quality of the joint by observing its internal structure.

●Macrometallographic examination:
▶Purpose: Inspect the weld’s penetration depth, weld width, weld bead shape, and the presence of macroscopic defects such as porosity, slag inclusions, incomplete penetration, and cracks.

▶Method: Cut, grind, and polish the weld cross section, then etch it with a specific etchant and observe with the naked eye or a low-power magnifying glass.

▶Problems that can be detected: Poor weld formation and various macroscopic welding defects.

●Micrometallographic examination:
▶Purpose: Observe the microstructure (such as austenite, ferrite, martensite, etc.) of various areas of the weld joint under a microscope to analyze the relationship between structure and performance.

▶Method: Observe the macrometallographic specimen using a high-magnification optical microscope or electron microscope.

▶Problems that can be detected: Microscopic defects such as harmful phase precipitation, coarse grains, and microcracks.

③ Fracture Toughness Test

For large, critical structures (such as nuclear power plants, ships, and pressure vessels), their ability to resist crack propagation needs to be evaluated.

● CTOD Test: Measures the crack tip opening displacement.

● J-Integral Test: Measures the stress-strain field intensity at the crack tip in elastic-plastic materials.

▶ Purpose: Provides key data for fracture prevention design and safety assessment of critical structures.

④ Chemical Analysis

▶ Purpose: Analyze the chemical composition of the weld metal to ensure it meets requirements, particularly for elements that significantly affect corrosion resistance (such as Cr, Ni, and C).

▶ Method: Spectroscopic analysis is typically used.

三. Destructive welding test：Experimental Procedure Overview

① Sampling: Cut specimens from the weld test plate or product at the specified location and orientation.

② Sample Preparation: Process the specimens into the size and shape specified by the standard.

③ Testing: Conduct testing using appropriate testing equipment.

④ Testing and Recording: Record test data (such as load, displacement, impact energy, etc.) and the specimen failure morphology.

⑤ Analysis and Reporting: Compare test results with standards or technical requirements, and issue an assessment report.

四. Destructive Testing vs. Nondestructive Testing

These are two complementary approaches to quality control: