High-Alloy Cr-Mo Steel Plate for High-Temperature Pipes: Microstructure and Mechanical Properties

Abstract

This study examines the production process, associated challenges, implemented countermeasures, and performance characteristics of high-alloy Cr-Mo steel plates for high-temperature-resistant steel pipes. Excellent physical quality was achieved through an innovative composition design, reduction of harmful elements during smelting, control of overheating during casting, precise regulation of heating temperature and holding time, strict billet preheating, rapid hot rolling, and careful avoidance of water after rolling. In addition, during quenching furnace heating, the temperature difference between the upper and lower surfaces of the steel plate was maintained at ≤5 °C. The composition, microstructure, and mechanical properties of the steel plate were investigated. The results indicate that the steel plate exhibits excellent tensile strength, a uniform, fine-grained microstructure, and overall superior mechanical performance.

Introduction

SA387Gr5 and SA387Gr9 steels are primarily used to manufacture high-temperature-resistant components for power plants and related equipment. They possess sufficient creep rupture strength, good heat transfer properties, high oxidation resistance, and excellent long-term structural stability. Boiler steels, both domestically and internationally, are generally classified into two categories: ferritic and austenitic. Ferritic heat-resistant steels are widely used in supercritical thermal power generation units. Compared to austenitic steels, ferritic steels have become the preferred material for boiler tubes due to their superior overall performance. The development of high-Cr-content ferritic heat-resistant steels, such as SA387Gr5 and SA387Gr9, has followed two main directions: increasing the Cr content and adding alloying elements such as Nb, V, Mo, W, and Co. A Cr content of 9%–12% is generally considered high-Cr, representing the highest grade of ferritic heat-resistant steel. This paper presents the trial production and performance testing of high-alloy Cr-Mo steel plates, aimed at verifying whether the plates meet the requirements for use in high-temperature-resistant steel pipes. The study provides data support and theoretical guidance for subsequent process optimization.

1. Technical Requirements

1.1 Test Materials and Methods

The trial production process for high-alloy Cr-Mo steel plates was as follows: primary refining → LF furnace refining → VD furnace vacuum degassing → wire feeding → casting → cleaning → heating → cogging → cleaning → heating → plate rolling → normalizing → tempering → plate finishing → sampling → mechanical testing → flaw detection → cutting to length → warehousing.

The as-supplied steel plates are in the normalized and tempered condition. Their chemical composition and mechanical properties are presented in Tables 1 and 2.

Table 1 Chemical Composition (mass fraction, %) of Trial-Produced High-Alloy Cr-Mo Steel Plates

Table 2 Mechanical Properties of Trial-Produced High-Alloy Cr-Mo Steel Plates

1.2 NDT Requirements

The trial-produced steel plates underwent 100% ultrasonic testing in accordance with ASME SA578/SA578M standards and met Acceptance Level C.

2. Production Difficulties and Countermeasures

High-alloy Cr-Mo steel plates present several production challenges: the high content of alloying elements makes composition control during smelting difficult. Overheating during casting is difficult to control accurately; steel ingots are hard to clean and cut, making them prone to cracks and edge bursting; and strict flatness requirements, particularly for thin-gauge plates, make flatness control even more challenging.

To address these challenges, the following countermeasures were adopted:

• Strict Composition Control
  Internal control standards were established to minimize fluctuations in chemical composition and reduce harmful elements. The main alloying elements of the steel plates were maintained within these control limits to ensure consistent and stable performance. Given the high alloying content, the smelting process of the molten steel was strictly controlled to ensure the quality of the ingots.
• Optimized Billet Heating, Cutting, and Cleaning
  During billeting and rolling, sufficient heating time was ensured. After billeting, the billets were transferred to a slow-cooling pit for preheating. Following preheating, warm cutting and cleaning were carried out. All surface defects were thoroughly and effectively removed to prevent cracks during rolling. For SA387Gr9CL2 steel, which has a high alloying content (Cr + Mo ≥ 10%), specialized cleaning and cutting procedures were implemented. The billets were loaded into the continuous furnace while still warm, and rolling was performed longitudinally without trimming or cross-cutting.
• Controlled Rolling and Flatness Management
  Warm rolling was performed rapidly. Apart from essential water cooling, no additional water was applied to prevent warping caused by uneven temperature distribution. After rolling, extra straightening passes were carried out in the straightening mill. During heating in the quenching furnace, the temperature difference between the upper and lower surfaces of the plate was maintained at ≤5 °C. During air cooling, rocking was increased to minimize surface temperature differences and roller marks, thereby ensuring both surface quality and flatness.

3. Product Quality Level

3.1 Chemical Composition

Through optimization of the steelmaking process, precise control of the chemical composition was achieved. The chemical composition of the trial-produced steel plates is shown in Table 3. As shown, the elemental contents meet both the design and standard requirements, with harmful elements such as P and S kept at very low levels. This precise compositional control ensures clean steel quality.

Table 3. Chemical Composition of High-Alloy Cr-Mo Steel Plates (Mass Fraction, %)

3.2 Mechanical Properties

To assess the mechanical properties of high-alloy Cr-Mo steel plates, samples were taken from various batches of SA387Gr5CL1 and SA387Gr9CL2 steel plates at the quarter-thickness position. The results are summarized in Tables 4 and 5. As shown, the mechanical properties at the quarter-thickness position fully meet the design standards. For the SA387Gr5CL1 steel plate, the yield strength (Rp0.2) ranged from 338 to 469 MPa, and the tensile strength (Rm) ranged from 489 to 550 MPa. For the SA387Gr9CL2 steel plate, the yield strength (Rp0.2) ranged from 490 to 545 MPa, with a tensile strength (Rm) of 660 to 686 MPa. For both grades, the elongation (A) was ≥25% and the reduction of area (Z) was ≥59%, demonstrating a sufficient safety margin and stable mechanical properties that fully meet the design requirements.

Table 4. Mechanical Properties of SA387Gr5CL1 Steel Plate

Table 5. Mechanical Properties of SA387Gr9CL2 Steel Plate

3.3 Ultrasonic Testing

All steel plates were subjected to 100% ultrasonic testing in accordance with ASME SA578/SA578M standards, achieving a passing grade of C. All 15 sampled plates met the specified requirements.

3.4 Metallographic Structure

After heat treatment, research samples were prepared from the SA387Gr5CL1 and SA387Gr9CL2 steel plates. The metallographic structure at the quarter-thickness position was examined using microscopy, and the results are presented in Figure 1.

Figure 1. Metallographic Morphology of Heat-Treated Trial Steel Plate at the Quarter-Thickness Position

Figure 1 clearly shows that the microstructure of the trial SA387Gr5CL1 and SA387Gr9CL2 steel plates is composed of uniform, fine, and evenly distributed bainite. Bainite is a medium-temperature transformation product of undercooled austenite, with retained austenite and carbides typically dispersed throughout the bainitic ferrite matrix. This microstructure is likely due to the relatively thin steel plates and the lower air-cooling temperature during normalizing, which led to a rapid cooling rate. As a result, carbide precipitation is minimal and uniformly distributed, with no segregation of alloying elements.

4. Conclusions

Through optimized composition design and process improvements, the elemental content of the produced steel plates falls within the specified technical requirements. 

• The chemical composition is well-designed, the mechanical properties are excellent, and the steel plates meet the C-level flaw detection standard of ASME SA578/SA578M. Their surface quality is high and fully satisfies user requirements. 
• The microstructure of SA387Gr5CL1 and SA387Gr9CL2 steel plates is bainitic, uniform, fine, and well-dispersed. Carbides are evenly distributed, with no segregation of alloying elements.