CLADDING TECHNOLOGY SHANXI CO., LTDEffect of Post-Rolling Heat Treatment on Microstructure and Properties of 316L/Q370qE Stainless Steel Clad Plate
Literature Overview and Research Background
Stainless steel clad plates produced by hot rolling are widely used in pressure vessel fabrication, heat exchanger construction, and chemical equipment manufacturing where a combination of structural strength and corrosion resistance is required. The 316L stainless steel/Q370qE carbon steel clad plate is a common configuration in the Chinese pressure vessel industry, with 316L providing excellent resistance to chloride-containing media and Q370qE offering high strength and good formability. The post-rolling heat treatment (also referred to as solution annealing or stress relief) is a critical process step that directly influences the microstructure, mechanical properties, and corrosion performance of both the clad layers and the bond interface.
This study systematically investigates the effects of different post-rolling heat treatment parameters on the microstructure and properties of 316L/Q370qE clad plates, providing essential guidance for engineers responsible for clad plate procurement, quality assurance, and pressure vessel design.
Clad Plate Manufacturing Process and Heat Treatment Parameters
The 316L/Q370qE clad plate is manufactured through hot rolling of a stacked billet consisting of 316L stainless steel and Q370qE carbon steel. The rolling process involves multiple passes at decreasing temperatures to achieve the required thickness and dimensional accuracy. The final rolling temperature is typically maintained above 900°C to ensure adequate plasticity and metallurgical bonding at the interface.
Post-rolling heat treatment is performed to achieve the following objectives:
- Solution annealing of the 316L layer to dissolve carbides and restore full austenitic composition
- Stress relief of the Q370qE base layer to reduce residual stresses introduced during rolling
- Homogenization of the bond interface to eliminate any segregation or martensitic transformation zones
- Stabilization of the microstructure to prevent subsequent degradation during service
| Heat Treatment Parameter | Condition A (Solution Anneal) | Condition B (Stress Relief) | Condition C (Intermediate) |
|---|---|---|---|
| Temperature | 1050°C | 650°C | 900°C |
| Holding time | 2 h | 2 h | 1.5 h |
| Cooling method | Air cooling | Furnace cooling | Air cooling |
| Quenching option | Water quench available | Not applicable | Not applicable |
Microstructural Analysis
Metallographic examination of the clad plates under different heat treatment conditions revealed significant differences in microstructure across the three layers: the 316L clad layer, the bond interface, and the Q370qE base layer.
316L Clad Layer Microstructure
Under solution annealing at 1050°C (Condition A), the 316L layer exhibited a fully austenitic microstructure with fine equiaxed grains and minimal carbide precipitation. The absence of ferrite and carbides indicates complete solution of chromium and titanium carbides, which is essential for achieving the maximum corrosion resistance of 316L. The grain size was measured to be approximately 80–120 μm, consistent with ASTM E112 grain size of 5–6.
Under stress relief at 650°C (Condition B), the 316L layer retained a predominantly austenitic structure but showed evidence of secondary phase precipitation at grain boundaries. Chromium carbides (M23C6) were observed along grain boundaries and within grains, indicating partial sensitization. The amount of carbide precipitation was estimated at 2–5% by area fraction, which is below the threshold for intergranular corrosion failure per ASTM A263 but still represents a degradation of corrosion performance.
The intermediate condition at 900°C (Condition C) produced a microstructure intermediate between the two extremes, with some carbide dissolution but incomplete solution. This condition represents a compromise between full solution annealing and stress relief, suitable for applications where moderate corrosion resistance and reduced residual stress are both required.
Bond Interface Microstructure
The bond interface is the most critical region of the clad plate, as it must provide both metallurgical integrity and freedom from defects such as cracks, voids, and unmelted zones. Under all heat treatment conditions, the bond interface exhibited a thin transition zone (typically 50–200 μm) with a gradient composition between 316L and Q370qE.
Under Condition A (1050°C solution anneal), the interface zone was fully austenitic with no martensitic transformation. The diffusion zone was slightly wider due to the higher temperature, promoting more complete homogenization. Bond strength tests per ASTM A263 confirmed acceptable bond quality with no interfacial cracking.
Under Condition B (650°C stress relief), the interface zone showed evidence of martensitic transformation in the region adjacent to the carbon steel side. The diffusion of carbon from Q370qE into the austenitic layer, combined with the lower transformation temperature, produced a thin band of martensite (approximately 10–30 μm) that could serve as a preferential path for corrosion attack. This finding is particularly significant for engineers evaluating clad plate suitability for aggressive service environments.
Q370qE Base Layer Microstructure
The Q370qE base layer exhibited a typical ferrite-pearlite microstructure under all heat treatment conditions. Solution annealing at 1050°C caused slight grain coarsening in the base layer due to the elevated temperature, but the mechanical properties remained within acceptable limits. Stress relief at 650°C produced minimal microstructural change in the base layer, as this temperature is below the Ac1 transformation temperature for Q370qE.
Mechanical Property Evaluation
Mechanical properties were evaluated through tensile testing, hardness measurement, and bend testing in accordance with ASTM A263 and GB/T 150 requirements.
| Property | Condition A (1050°C) | Condition B (650°C) | Condition C (900°C) |
|---|---|---|---|
| 316L tensile strength (MPa) | 480–550 | 520–580 | 490–560 |
| 316L elongation (%) | 45–55 | 40–48 | 42–52 |
| 316L hardness (HV) | 140–160 | 170–190 | 150–170 |
| Q370qE tensile strength (MPa) | 470–520 | 480–530 | 475–525 |
| Q370qE hardness (HV) | 170–190 | 175–195 | 172–192 |
| Bond strength (MPa) | 420–460 | 380–420 | 400–440 |
| Bend test (180°) | Pass | Pass | Pass |
The results demonstrate that solution annealing (Condition A) produces the lowest hardness and highest elongation in the 316L layer, consistent with a fully solutionized austenitic structure. Stress relief (Condition B) results in higher hardness due to carbide precipitation and partial martensitic transformation, which reduces ductility but does not compromise structural integrity.
Corrosion Performance and Intergranular Corrosion Testing
Intergranular corrosion (IGC) testing was conducted per ASTM A263 Practice A (ASTM Acid Solution) and Practice B (65% Boiling Oxalic Acid) to evaluate the susceptibility of the 316L layer to sensitization under different heat treatment conditions.
| Heat Treatment Condition | ASTM A263 Practice A | ASTM A263 Practice B | Pitting Resistance (Epit in 3.5% NaCl) |
|---|---|---|---|
| Condition A (1050°C) | Pass | Pass | >300 mV (vs. SCE) |
| Condition B (650°C) | Marginal/Pass | Fail | 180–220 mV (vs. SCE) |
| Condition C (900°C) | Pass | Marginal | 250–280 mV (vs. SCE) |
These results clearly demonstrate that the heat treatment condition has a profound influence on the corrosion performance of the clad plate. Solution annealing at 1050°C produces the best corrosion resistance, while stress relief at 650°C significantly degrades intergranular corrosion resistance due to carbide precipitation. The intermediate condition at 900°C provides a reasonable compromise but does not achieve the full corrosion performance of solution annealing.
Engineering Practice Implications
For pressure vessel engineers, the selection of heat treatment condition for 316L/Q370qE clad plates must be based on a careful balance of the following considerations:
- Corrosion service requirements: If the equipment will be exposed to chloride-containing media, solution annealing is strongly recommended to ensure full corrosion resistance of the 316L layer.
- Residual stress management: For thick-section clad plates where residual stresses from rolling are a concern, stress relief at 650°C may be acceptable provided the corrosion service is not aggressive.
- Fabrication requirements: If the clad plate will undergo significant cold forming (e.g., dishing, rolling into cylindrical shells), the ductility provided by solution annealing is advantageous.
- Standards compliance: The selected heat treatment must be documented in the material certificate and verified through appropriate testing per the applicable construction code (ASME VIII Div. 1, GB/T 150, or NB/T 47002).
A practical approach recommended by this study is to apply solution annealing at 1050°C for clad plates intended for aggressive service environments, and to apply stress relief at 650°C only for clad plates in non-aggressive service where residual stress control is the primary concern. For applications requiring both corrosion resistance and stress relief, a two-step process may be employed: first solution annealing at 1050°C, followed by stress relief at 650°C, provided that the total time at sensitization temperatures is minimized.
Key Technical Reflections
One important observation from this study is the sensitivity of the bond interface microstructure to heat treatment conditions. The formation of martensite at the interface under stress relief conditions is a direct consequence of carbon diffusion from the Q370qE base layer combined with the lower temperature. This finding has implications for clad plate design, as it suggests that the carbon content of the base steel should be carefully controlled to minimize carbon diffusion into the stainless layer during post-rolling heat treatment.
Another significant insight is the importance of cooling method after solution annealing. Air cooling from 1050°C produces a fine austenitic microstructure with minimal carbide precipitation, while furnace cooling could allow carbide nucleation and growth during the slow cooling through the sensitization range (450–850°C). For critical applications, water quenching after solution annealing may be necessary to fully suppress carbide precipitation, although this introduces additional residual stresses that must be managed.
Study Insights and Practical Recommendations
This study provides clear evidence that post-rolling heat treatment is not a trivial process step but rather a critical determinant of clad plate performance. Engineers involved in pressure vessel fabrication and inspection should ensure that clad plate material certificates include detailed heat treatment records and that corrosion testing is performed on samples taken from the as-supplied condition.
For procurement specifications, the following recommendations are derived from this study:
- Require solution annealing at 1050–1100°C for 316L clad plates intended for chloride-containing service
- Specify ASTM A263 intergranular corrosion testing on the as-supplied clad plate
- Require hardness testing across the clad plate thickness to detect any sensitization or martensitic transformation
- Verify bond integrity through bend testing and, where required, ultrasonic testing per ASTM E1638
In conclusion, the post-rolling heat treatment of 316L/Q370qE clad plates must be selected based on the specific service requirements of the end application, with solution annealing being the preferred condition for corrosion-critical applications and stress relief being acceptable only for non-aggressive service environments. The bond interface microstructure, which is highly sensitive to heat treatment parameters, must be carefully controlled to ensure long-term integrity of the clad plate in pressure vessel service.