Effect of Solution Temperature on the Microstructure and Mechanical Properties of Ultra-Low Carbon 15-5PH Precipitation Hardening Stainless Steel

Effect of Solution Temperature on the Microstructure and Mechanical Properties of Ultra-Low Carbon 15-5PH Precipitation Hardening Stainless Steel

Introduction

15-5PH (05Cr15Ni5Cu4Nb) stainless steel is a martensitic precipitation hardening stainless steel known for its high strength, good consistency of mechanical properties in both longitudinal and transverse directions, simple heat treatment process, and corrosion resistance. It is widely used in aerospace, shipbuilding, weaponry, and petroleum machinery industries for components requiring high strength, high toughness, and good corrosion resistance.

The high strength and toughness of 15-5PH stainless steel are achieved through solution heat treatment followed by quenching to induce martensitic transformation. Subsequent aging treatment precipitates strengthening phases such as ε-Cu, Nb, and C. Previous studies have investigated the effects of carbon content, solution treatment time, and aging temperature on the microstructure and properties of 15-5PH stainless steel. However, the influence of solution treatment temperature on ultra-low carbon (ω(C) ≤ 0.01%) 15-5PH stainless steel remains unexplored. This study aims to bridge this gap and provide a theoretical basis for its engineering application and heat treatment optimization.

1. Experimental Materials and Methods

1.1. Experimental Materials

The experimental material used was a 5-ton ultra-low carbon 15-5PH stainless steel ingot smelted by vacuum induction melting followed by vacuum consumable melting. The chemical composition (wt.%) was: 0.005C, 0.45Si, 0.55Mn, 0.004S, 0.005P, 15.45Cr, 5.40Ni, 0.46Mo, 3.80Cu, and 0.40Nb. The ingot was heated to 1200°C for forging and rolling to obtain a bar with a diameter of 45 mm. After annealing, longitudinal samples were cut from the bar end for physicochemical and mechanical testing.

1.2. Experimental Methods

Longitudinal tensile, impact, and metallographic samples were prepared from the bar segments. The samples were solution treated at 975°C, 1000°C, 1025°C, and 1050°C for 1 hour in a muffle furnace followed by air cooling. Subsequently, aging treatment was performed at 480°C for 1 hour followed by air cooling. Tensile tests and Charpy impact tests were conducted according to GB/T 228.1—2021 and GB/T 229—2020, respectively. Microstructural analysis was carried out using a ZEISS Axio Imager optical microscope. Fracture surface morphology was observed using a ZEISS Ultr55 scanning electron microscope. EBSD analysis was performed using a Merlin compact scanning electron microscope and Channe5 data processing software.

2. Results and Discussion

2.1. Mechanical Properties

The mechanical properties of ultra-low carbon 15-5PH stainless steel after solution treatment at different temperatures and aging at 480°C are shown in Figure 1. The solution temperature significantly affected the mechanical properties. With increasing solution temperature, the plasticity and toughness decreased significantly, while the tensile and yield strength remained relatively stable. The elongation decreased from 14.5% to 8.5%, reduction in area decreased from 55% to 32%, and impact energy decreased from 35.6 J to 6.6 J as the solution temperature increased from 975°C to 1050°C.

2.2. Fracture Morphology

The tensile fracture surfaces of the samples solution treated at different temperatures exhibited a typical ductile fracture morphology with a dimpled structure (Figure 2). However, the size and depth of the dimples varied significantly. Samples solution treated at lower temperatures (975°C and 1000°C) showed a higher density of deep and fine dimples, indicating higher ductility. In contrast, samples solution treated at higher temperatures (1025°C and 1050°C) exhibited fewer dimples and larger areas of cleavage fracture, indicating lower ductility.

Similar trends were observed in the impact fracture surfaces (Figure 3). Samples solution treated at lower temperatures exhibited a larger fibrous zone with a higher density of fine dimples, indicating higher impact toughness. As the solution temperature increased, the fibrous zone became smaller and narrower, and the proportion of cleavage fracture increased, indicating lower impact toughness.

2.3. Microstructural Analysis

Optical microscopy revealed that the microstructure of all samples consisted of tempered martensite with different orientations (Figure 4). However, the prior austenite grain size increased significantly with increasing solution temperature. The average grain size increased from 27 μm to 150 μm as the solution temperature increased from 975°C to 1050°C (Figure 5).

EBSD analysis confirmed the increase in grain size with increasing solution temperature (Figure 6). The average effective grain size, which is considered to be closely related to toughness, increased from 8.71 μm to 23.97 μm as the solution temperature increased from 975°C to 1050°C.

Furthermore, EBSD analysis revealed that the proportion of high-angle grain boundaries (HAGBs, θ > 15°) decreased with increasing solution temperature (Figure 7). HAGBs are known to impede crack propagation more effectively than low-angle grain boundaries. The decrease in HAGB fraction with increasing solution temperature is consistent with the observed decrease in toughness.

3. Conclusions

(1) The solution treatment temperature significantly affects the plasticity and toughness of ultra-low carbon 15-5PH stainless steel. Increasing the solution temperature leads to a decrease in both plasticity and toughness.

(2) The solution treatment temperature also significantly affects the grain size and grain boundary characteristics. Increasing the solution temperature results in grain growth and a decrease in the proportion of high-angle grain boundaries.

(3) The grain size of ultra-low carbon 15-5PH stainless steel has a significant effect on its plasticity and toughness. Smaller grain sizes lead to improved plasticity and impact toughness, while larger grain sizes result in decreased plasticity and impact toughness.

These findings suggest that a lower solution treatment temperature (e.g., 975°C) is beneficial for achieving optimal mechanical properties in ultra-low carbon 15-5PH stainless steel.