Blowout Preventers (BOPs) are critical well control equipment used in drilling, workover, and well testing operations to manage wellbore pressure. As core components in deep oil and gas drilling, BOP steel castings are typically made of medium-high carbon low-alloy high-strength steel. While material properties can be optimized through composition adjustments and heat treatment, improper welding repairs on defective castings may cause severe hardness reduction, coarse grains, and degraded mechanical performance, compromising operational safety. This study evaluates two welding processes for BOP steel castings under different heat treatment conditions, validated against API 16A and NACE MR0175/ISO 15156 standards.
1. Material Characteristics and Weldability
The BOP housing castings studied utilize ASTM A487 Grade 4 steel with the chemical composition shown in Table 1. The carbon equivalent (CE) calculated using the International Institute of Welding (IIW) formula demonstrates significant hardening tendency:
$$CE = C + \frac{Mn}{6} + \frac{Cr + Mo + V}{5} + \frac{Ni + Cu}{15} = 0.77\%$$
This high CE value (>0.4%) indicates poor weldability due to martensite formation susceptibility. Table 2 compares electrode compositions for Process 1 (post-weld quenching/tempering) and Process 2 (welding in quenched/tempered condition).
| Element | C | Si | Mn | Cr | Ni | Mo |
|---|---|---|---|---|---|---|
| Content | 0.25-0.33 | 0.4-0.7 | 0.8-1.2 | 0.5-1.0 | 0.4-0.8 | 0.25-0.45 |
| Process | C | Si | Mn | Cr | Ni | Mo |
|---|---|---|---|---|---|---|
| Process 1 (E10015-G) | 0.13 | 0.42 | 1.59 | 1.62 | 0.51 | 0.55 |
| Process 2 (E9015-B3) | 0.083 | 0.22 | 0.83 | 2.25 | 0.05 | 1.06 |
2. Welding Process Design
Both processes aim to maintain hardness uniformity (207-237 HBW) across weld zones while achieving required toughness (≥42 J at -29°C). Key parameters are compared in Table 3.
| Parameter | Process 1 | Process 2 |
|---|---|---|
| Preheat Temperature | 180°C | 150°C |
| Interpass Temperature | 200-350°C | 150-200°C |
| Heat Input | 1.2-2.4 kJ/mm | 0.8-1.5 kJ/mm |
| Post-Weld Heat Treatment | Quenching (870°C) + Tempering (620°C) | Stress Relief (620-670°C) |
2.1 Process 1: Post-Weld Quenching/Tempering
Using composition-matched electrodes (CE = 0.75%), this process employs multi-layer welding with controlled heat input:
$$Q = \frac{60 \times I \times V}{1000 \times S}$$
Where Q = heat input (kJ/mm), I = current (A), V = voltage (V), and S = travel speed (mm/min). The slow cooling rate during tempering (≤60°C/h) prevents residual stresses.
2.2 Process 2: Welding in Q&T Condition
This process uses higher CE electrodes (0.82%) with reduced heat input to minimize HAZ softening. The martensite volume fraction can be estimated by:
$$MVF = 1 – \exp(-0.011(MS – T_{\text{cool}}))$$
Where MS = martensite start temperature (~320°C) and Tcool = cooling time between 800-500°C.
3. Performance Evaluation
Both processes meet API 16A requirements as shown in Table 4. Process 1 demonstrates better hardness uniformity due to complete phase transformation during post-weld heat treatment.
| Property | Standard | Process 1 | Process 2 |
|---|---|---|---|
| Tensile Strength | ≥655 MPa | 682-693 MPa | 677-685 MPa |
| Yield Strength | ≥551 MPa | 564 MPa | 562 MPa |
| Impact (-29°C) | ≥42 J | 64-119 J | 56-133 J |
| Hardness Range | 207-237 HBW | 207-225 HBW | 207-249 HBW |
4. Microstructural Analysis
Process 2 exhibits mixed martensite-bainite-ferrite microstructure in weld metal (Figure 1a) and tempered martensite in HAZ (Figure 1b). The hardness differential between weld (210-249 HV) and base metal (207-237 HV) is controlled through:
$$\Delta HV = 0.25(C_{\text{weld}} – C_{\text{base}}) \times 100 + 12(Mn_{\text{weld}} – Mn_{\text{base}})$$
Where C and Mn represent weight percentages of carbon and manganese.
5. Process Selection Guidelines
For steel casting repairs:
- Use Process 1 (with full heat treatment) for major defects (>5% wall thickness)
- Apply Process 2 (local stress relief) for minor defects (<2% wall thickness)
The maximum allowable repair depth follows:
$$D_{\text{max}} = 0.15t + 1.5\ \text{(mm)}$$
Where t = casting wall thickness (mm).
6. Conclusion
Both welding processes effectively maintain the mechanical integrity of BOP steel castings when properly implemented. Process 1 achieves better homogeneity through complete phase transformation, while Process 2 offers efficiency for minor repairs. The critical factors for successful steel casting repair include:
- CE matching between base metal and electrode
- Precise control of interpass temperature
- Post-weld heat treatment parameter optimization
These methodologies ensure compliance with industry standards while addressing the unique challenges of high-strength steel casting repairs in critical well control equipment.