Abstract
This article delves into the challenges faced in ensuring the plasticity of copper-containing steel castings, which are increasingly utilized in marine and shipbuilding applications due to their superior strength, plasticity, low-temperature impact toughness, and weldability. We present a comprehensive case study involving a large copper-containing steel casting that failed to meet plasticity requirements despite adhering to specified chemical composition and heat treatment protocols. By conducting a detailed examination encompassing fracture morphology, microstructural analysis, and chemical composition verification, we identify the root cause of the plasticity issue and propose effective improvement measures. The implementation of these measures, supported by theoretical hydrogen diffusion calculations and practical validation, successfully enhanced the plasticity of the steel casting.
1. Introduction
Copper-containing steel castings have gained widespread popularity in recent years due to their exceptional mechanical properties, making them ideal for demanding applications such as offshore platforms and ships. However, achieving consistent high-quality castings can be challenging, particularly with regard to ensuring adequate plasticity. In this study, we focus on a specific case where a copper-containing steel casting exhibited inadequate plasticity despite meeting other mechanical property requirements.
2. Case Description
A large copper-containing steel casting with a maximum wall thickness of 300 mm was manufactured using an electric arc furnace followed by LF+VD refining. After heat treatment, the casting underwent mechanical testing, revealing that while its yield strength, ultimate tensile strength, and impact toughness satisfied technical specifications, the elongation after fracture fell short of the required minimum value.
3. Materials and Methods
3.1 Material Specification
The copper-containing steel casting was produced from a material with the following specified chemical composition (Table 1).
| Element | Specification (%) | Measured (%) |
|---|---|---|
| C | ≤ 0.13 | 0.11 |
| Si | ≤ 0.50 | 0.18 |
| Mn | ≤ 0.80 | 0.76 |
| P | ≤ 0.020 | 0.008 |
| S | ≤ 0.030 | 0.005 |
| Ni | ≤ 1.60 | 1.45 |
| Cr | ≤ 0.30 | 0.26 |
| V | ≤ 0.10 | 0.002 |
| Cu | ≤ 0.80 | 0.70 |
Table 1: Chemical composition of the copper-containing steel casting
3.2 Heat Treatment
The casting underwent a standard heat treatment protocol, including normalizing, quenching, and tempering, with temperatures and times specified in Table 2.
| Process Step | Temperature (°C) | Time (h) | Cooling Method |
|---|---|---|---|
| Preheating | 640 ± 10 | 4 | Furnace cooling |
| Normalizing | 920 ± 10 | 8 | Air cooling |
| Tempering | 620 ± 10 | 7 | Air cooling |
Table 2: Heat treatment protocol for the copper-containing steel casting
3.3 Testing Methods
Tensile and impact tests were conducted according to GB/T 228 and GB/T 229 standards, respectively. Tensile specimens were Ø10 mm in diameter, while impact specimens conformed to the standard Charpy V-notch configuration.
4. Experimental Results
4.1 Mechanical Properties
While the casting met the requirements for yield strength, ultimate tensile strength, and impact toughness, the elongation after fracture fell below the specified minimum of 20% (Table 3).
| Property | Requirement | Measured Value |
|---|---|---|
| ReL (MPa) | ≥ 370 | 406 |
| Rm (MPa) | ≥ 490 | 526 |
| A (%) | ≥ 20 | 18.0 |
| Z (%) | ≥ 40 | 57 |
| -40°C KV2 (J) | ≥ 27 | 45 |
| Average -40°C KV2 | ≥ 27 | 43 |
Table 3: Mechanical properties of the copper-containing steel casting
4.2 Fracture Morphology
Fractographic analysis of the tensile specimen revealed a lack of typical plastic deformation features such as necking and shear lips. Instead, numerous circular dimples with smooth, quasi-cleavage fracture surfaces were observed, indicative of hydrogen-induced embrittlement.
4.3 Microstructural Analysis
Microstructural examination of the impact specimen using optical microscopy revealed a mixture of ferrite, pearlite, and a small amount of tempered bainite. Microsegregation regions were evident, with pearlite and bainite distributed around ferrite dendrites.
4.4 Chemical Analysis
Chemical analysis confirmed that the casting’s elemental composition adhered to the specified limits (Table 1). However, hydrogen content measurements revealed an abnormally high value of 5.5 ppm, significantly exceeding the expected level based on the melting analysis report (1.5 ppm).
5. Discussion
5.1 Root Cause Analysis
The high hydrogen content and presence of microscopic voids in the casting indicated hydrogen embrittlement as the primary cause of the reduced plasticity. The high hydrogen levels likely originated from the pouring process, during which argon protection was temporarily compromised due to an argon tube detachment. Additionally, insufficient sand mold baking during rainy weather may have contributed to elevated hydrogen pickup.
5.2 Hydrogen Diffusion Calculations
To mitigate the hydrogen embrittlement issue, extended dehydrogenation heat treatment was proposed. Using the Fick’s second law of diffusion, hydrogen diffusion coefficients, and casting dimensions, we calculated the expected hydrogen concentrations after various dehydrogenation times (Table 4).
| Dehydrogenation Time (h) | F0 | Bi | rR | U | Remaining Hydrogen Content (ppm) |
|---|---|---|---|---|---|
| 72 | 0.2475 | 6 | 0 | 0.5466 | 3.0 |
| 144 | 0.4950 | 6 | 0 | 0.1918 | 1.0 |
| 216 | 0.7426 | 6 | 0 | 0.0731 | 0.4 |
Table 4: Hydrogen diffusion calculations for various dehydrogenation times
Based on the calculations and considering the maximum allowable hydrogen content (≤ 1.8 ppm), a dehydrogenation time of 144 hours was selected for implementation.
6. Improvement Measures
To address the identified hydrogen embrittlement issue, the following improvement measures were undertaken:
- Extended Dehydrogenation Heat Treatment: The dehydrogenation time was increased to 144 hours to effectively reduce hydrogen content below the critical limit.
- Enhanced Pouring Protection: Stricter argon protection protocols were implemented during pouring to prevent hydrogen ingress.
- Improved Sand Mold Baking: Sand molds were thoroughly baked to ensure complete moisture removal, further minimizing hydrogen pickup.
- Strict Quality Control: Regular inspections and maintenance of pouring equipment were enforced to prevent similar incidents in the future.
7. Verification Testing
After implementing the improved dehydrogenation heat treatment, the casting was resubjected to mechanical testing. The results indicated a marked improvement in plasticity, with elongation after fracture increasing from 18.0% to 25.0%, and an overall enhancement in ductility and impact toughness (Table 5).
| Property | Requirement | Measured Value After 144h Dehydrogenation |
|---|---|---|
| ReL (MPa) | ≥ 370 | 410 |
| Rm (MPa) | ≥ 490 | 538 |
| A (%) | ≥ 20 | 25.0 |
| Z (%) | ≥ 40 | 70 |
| -40°C KV2 (J) | ≥ 27 | 74 |
| Average -40°C KV2 | ≥ 27 | 72 |
Table 5: Mechanical properties of the copper-containing steel casting after improved dehydrogenation treatment
8. Conclusion
This study highlights the challenges associated with ensuring adequate plasticity in copper-containing steel castings and underscores the importance of stringent quality control measures to prevent hydrogen embrittlement. Through a combination of fracture morphology analysis, microstructural examination, chemical analysis, and theoretical hydrogen diffusion calculations, we successfully identified the root cause of the plasticity issue and developed effective improvement measures. The implementation of extended dehydrogenation heat treatment, coupled with enhanced pouring protection and sand mold baking, significantly improved the casting’s plasticity, ductility, and impact toughness, demonstrating the efficacy of our approach.