Experimental Study on the Variation Law of Stress Corrosion Threshold Value of 4130X Steel

Project Background

This project originates from a sub-project (2016YFC0801905) of the National Key Research and Development Program during China’s 13th Five-Year Plan titled “Predictive Life and Extension Key Technologies for Over-Serviced Pressure Equipment.” It focuses on researching the life prediction methods and safety assessment of over-serviced pressure equipment under non-steady-state conditions, specifically targeting typical materials used in such equipment.

Core Research Issues

This study centers on exploring the influencing factors of the stress corrosion threshold value KISCC of materials and establishing a mathematical model between KISCC and hydrogen sulfide concentration (CH2S).

Research Objectives

Based on fracture mechanics theory and taking 4130X steel used in gas cylinders as the research object, experimental studies were conducted to investigate the impact of hydrogen sulfide corrosion on the fracture toughness and stress corrosion threshold value of the material from two perspectives: pure corrosion and stress corrosion. The study particularly focused on the changes in the material’s stress corrosion threshold value under non-single conditions (different corrosive environments and applied stresses).

Methods and Results

To provide experimental evidence for predicting the service life under long-term operation, the study first examined the influence of pure corrosion on the fracture toughness of the material. Using three-point bending specimens and COD (crack opening displacement) tests, the fracture toughness of 4130X material was measured before and after corrosion in air and at 1600 ppm hydrogen sulfide concentration. A total of 21 P-V curves were obtained (air: 6 sets of raw material, 6 sets of heat-treated material; post-corrosion: 3 sets of raw material, 6 sets of heat-treated material), and corresponding dR−Δa relationships were derived.

For the heat-treated material, dR−Δa relationships were used to construct load resistance curves and calculate KI values. For the raw material, since there was no stable crack propagation phase, the upper limit of KI was determined.

Subsequently, improved WOL (Wedge Opening Load) specimens were used for constant displacement experiments to investigate the influence of corrosion and external force factors on the variation law of the stress corrosion threshold value. Experiments were performed considering three different hydrogen sulfide concentrations (200 ppm, 800 ppm, 1600 ppm) and three different initial loads, yielding 18 stress corrosion threshold values (raw material: 9 values, heat-treated material: 9 values).

Due to the low yield strength of the raw material specimens, the results did not satisfy plane strain requirements and represented the upper limit of the material’s stress corrosion threshold value.

Main Contributions and Innovations

  1. Through experiments, a mathematical model was established between the stress corrosion threshold value KISCC (or Ke) of 4130X material and hydrogen sulfide concentration CH2S. In engineering applications, when the hydrogen sulfide concentration is known, the stress corrosion threshold value can be predicted using this model.
  2. During the experiments, both hydrogen sulfide concentration and initial load were considered as influencing factors. The results indicated that hydrogen sulfide concentration is the primary factor affecting the stress corrosion threshold value, showing an exponential decrease with increasing hydrogen sulfide concentration. Initial load is a secondary factor, mainly influencing the stress corrosion threshold value at low concentrations and decreasing in significance with increased hydrogen sulfide concentration.
  3. Systematic experimental research was conducted on 4130X high-pressure gas cylinder steel, both in its raw and heat-treated states, focusing on mechanical properties, fracture behavior, corrosion resistance, and stress corrosion performance. Quantitative evaluation results suitable for engineering applications were provided.