How Composition, Structure, and Processing Shape 125ksi Super Martensitic Pipes

Oil and natural gas are vital national strategic resources. As exploration and production shift toward deepwater offshore fields, deep terrestrial formations, and unconventional resources, the demand for high-performance materials, particularly high-end specialty steels, has increased significantly. These demanding environments require steel materials with exceptional strength, corrosion resistance, and reliability.

To address this rising demand, World Metal Herald and Daye Special Steel, a subsidiary of CITIC Pacific Special Steel Group, have jointly released a special report titled “R&D and Application of High-End Special Steels for Oil and Gas.” This in-depth report highlights China’s progress in developing and applying advanced special steels for oil and gas exploration and production.

The report not only showcases key technological achievements but also looks ahead to future challenges and innovations. By supporting major domestic and international energy projects, it aims to contribute to the sustainable development of the global energy sector.

Optimizing 15Cr Performance

This paper analyzes and discusses the influence of composition design, heat treatment process, and smelting and hot working process parameters on strength-toughness matching and microstructure. This paper proposes a comprehensive composition-microstructure-performance control technology for 125ksi grade 15Cr oil pipe. To ensure a balanced strength-toughness ratio, composition design must consider factors such as delta ferrite phase control, while also adjusting the material heat treatment regime. My country is currently developing and accumulating strong-toughness-corrosion resistance matching technology and production experience for 15Cr super martensitic stainless steel, which is essential to breaking the foreign monopoly and enhancing the product’s international competitiveness.

1. Introduction

Oil-well pipes (OPPs) consist of drill string components, casing, and tubing, which are connected through specialized threaded connections to form the drill string, casing string, and tubing string. The drill string’s primary function is oil and gas development, while the casing and tubing strings provide the sole pathways for isolating formations and extracting oil and gas. Therefore, the life of the casing determines the life of the oil and gas well. Oil-well pipes are generally produced according to American Petroleum Institute (API) standards. Currently, my country has achieved domestic production of conventional API-standard tubing and casing, with products comparable to those produced abroad.

My country’s onshore oil and gas fields have complex geological conditions and deep burial depths, particularly in the Tarim and Sichuan oil fields in northwest China. The maximum well depth is nearly 9,000 meters, with downhole temperatures reaching 200°C and pressures as high as 150 MPa. These fields are often exposed to complex corrosive media such as CO₂, H₂S, and Cl₁₄, as well as geological environments such as piedmont structures and salt-gypsum formations. Conventional API products are unable to meet these service requirements, leading to the widespread adoption of non-API tubing with high corrosion resistance and high strength and toughness. When downhole temperatures are below 170°C, 95ksi-110ksi super 13Cr oilwell pipes are widely used. As well depths increase and downhole temperatures exceed 180°C, 125ksi-grade 15Cr oilwell pipes exhibit significantly lower corrosion rates in formation water and acidification environments than 13Cr. Therefore, replacing 13Cr in well sections with temperatures exceeding 180°C can better ensure wellbore safety.

JFE’s UHP1-15Cr-125 is a prime example of 15Cr oilwell pipe suitable for CO2 corrosion environments. It is currently the only commercially available product internationally. Its main components are 15Cr-6Ni-2Mo-1Cu, with a carbon content of no more than 0.04%. Sumitomo Corporation has also patented a 125ksi steel grade with a main composition of 17Cr-6Ni-2Mo-3Cu, and a carbon content of no more than 0.03%. Compared to 13Cr steel, 15Cr oil pipe boasts higher Cr, Mo, and Ni content and the addition of Cu, resulting in a higher strength grade. However, the increased complexity of the alloy system also increases the difficulty of controlling the composition, structure, properties, and process. This article describes the comprehensive control technology for the composition, structure, and properties of 125ksi steel-grade 15Cr oil pipe.

2. Composition Design

15Cr super martensitic stainless steel must meet the following mechanical property requirements for 125ksi steel: yield strength of 863-1034MPa, elongation of not less than 16%, longitudinal KV value of not less than 140J, and transverse KV value of not less than 120J at -10°C, and hardness not greater than 37HRC.

1) δ-Ferrite Phase Control

Increasing the Cr equivalent in the composition increases the tendency for δ-ferrite to form. Excessive δ-ferrite will form a banded structure, reducing transverse impact toughness and degrading corrosion resistance. Therefore, composition design must first consider the balance of Cr/Ni equivalents to avoid excessive ferrite formation. Thermodynamic calculations can be used to analyze the formation temperature and content of δ-ferrite at different Cr/Ni equivalents. For the calculation of other elements, C is set at 0.015%, Cr at 15.5%, Ni at 6.5%, and Cu at 1.25%. Thermodynamic phase diagrams show that Cr and Ni have a more pronounced effect on δ-ferrite, while C and Cu have a slightly lesser influence. When the Cr content exceeds 15%, delta ferrite transformation occurs within the hot working temperature range (around 1150°C). When the Cr content reaches 17%, delta ferrite reaches nearly 20% at 1150°C, and delta ferrite transformation also occurs within the solution treatment temperature range (950-1050°C). When the Ni content reaches 16%, delta ferrite transformation occurs within the solution treatment temperature range (950-1040°C), and over 10% delta ferrite forms within the hot working temperature range. Therefore, from the perspective of delta ferrite control, a Cr content of 15% is preferred over 17%. In this case, the Ni content should be 6.5%-7% to prevent excessive delta ferrite formation during hot working, heating, and holding.

2) Strength and Toughness Matching

Oil-well tubular goods require a high balance of strength and toughness to prevent failures such as bending due to insufficient stiffness when used in deep and ultra-deep well sections. On the other hand, they need to have sufficient toughness to enhance stress corrosion resistance. The increase in oil pipe steel grade to 125ksi has not reduced the requirements for plasticity and toughness, undoubtedly increasing the difficulty of controlling composition. The primary strengthening elements in 125ksi super martensitic stainless steel are C, Mo, and Cu, while the toughening element is primarily Ni. C and Mo primarily contribute to solid solution strengthening, while Cu primarily contributes to precipitation strengthening. The high alloying element content in 125ksi steel reduces the material’s Ms.. Therefore, the strength-toughness balance should be optimized through phase transformation. This aims to achieve a sufficient martensitic transformation at room temperature for sufficient strength while retaining an appropriate amount of austenite for improved toughness. Experimental research has shown that the matching of C and Cu elements has the most significant impact on phase transformation.

The experimental results demonstrate the effects of varying C and Cu elements on the strength and toughness of 15Cr material during quenching at 1040°C and heat treatment at 500-550°C. When the C content is within the range of 0.01%-0.03%, the Cu content of 1.0%-1.5% can meet the 125ksi steel grade requirements. When the Cu content is greater than 2.5%, the strength decreases significantly. Therefore, when the C content is within the range of 0.01%-0.03%, the Cu content should be controlled at 1.0%-1.5%. When the Cu content is 1.0%, the tempering temperature range of 525-550℃ can meet the strength and toughness requirements. The impact energy margin of tempering at 525℃ is not large, and the strength and toughness of tempering at 550℃ have a certain margin. When the Cu content is 1.25%, the strength does not change much, and the impact toughness is greatly improved. The strength and toughness requirements of the tempering range of 485-550℃ are met, and the margin is large. When the Cu content is 1.5%, the strength after tempering at 550℃ reaches the lower limit, and the 485-525℃ range has a high strength and toughness match. Therefore, when the C content is within the 0.01%-0.03% range, controlling the Cu content to 1.25%-1.5% achieves a better balance between strength and toughness, and also allows for a wider heat treatment temperature range. A 1040°C quench followed by a 500-550°C tempering heat treatment is recommended. At a 1% Cu content, lowering the C content, while strength decreases somewhat, significantly improves the -10°C impact energy. Tempering at 485-550°C also meets the strength and toughness requirements, but tempering at 550°C reaches its lower limit. Therefore, a 1040°C quench followed by a 485-525°C tempering heat treatment is recommended. Within the 1.0%-1.5% Cu content, lowering the C content can achieve a better balance between strength and toughness. However, this reduction in C content may increase the delta ferrite content, making industrial smelting more difficult. When the carbon content is less than 0.01%, a Cu content of 2.0%-2.5% can meet the strength requirements of a 125ksi steel grade. Tempering in the 485-550°C temperature range achieves the required strength-toughness balance. The impact energy margin at -10°C is very large, but the strength margin decreases when tempered at 550°C, so a tempering treatment of 485-525°C is recommended. A Cu content of 3.0% provides very high strength, but the -10°C impact toughness decreases significantly. Therefore, when the carbon content is less than 0.01%, the Cu content can be controlled to 2.0%-2.5%, and a tempering heat treatment of 485-525°C is recommended.

15Cr oil pipe material requires very high corrosion resistance. Reducing the carbon content can improve corrosion resistance. However, industrial production generally uses electric furnaces combined with off-furnace refining to control the carbon content to less than 0.01%, which is difficult to achieve and not conducive to cost control. Therefore, a carbon content of less than 0.01% is not recommended. It is recommended that the C content be controlled within the ultra-low range of 0.01%-0.03% and the Cu content be controlled within the range of 1.0%-1.5%. By adjusting the heat treatment process, an ideal balance of strength and toughness can be achieved.

After heat treatment with different C and Cu ratios, the retained austenite content in the steel was measured by XRD diffraction. The results are shown in Table 1. After heat treatment, the retained austenite content of 15Cr material increased with increasing C and Cu content, with C content having a greater impact than Cu. The retained austenite content of the test steels with a good balance of strength and toughness after heat treatment was approximately 10%-15%. The strength of the test steels with different compositions decreased with increasing austenite content, while toughness increased. A suitable martensite-austenite ratio ensures a good balance of strength and toughness, with a proper matching of C and Cu being the most critical.

Retained Austenite Volume Fraction After Heat Treatment of Test Steels with Different C and Cu Compositions

3. The Impact and Control of Impurity Elements in Electric Furnace Smelting

Oil-well tubular goods are typically smelted using an electric furnace combined with external refining. Continuous casting or die casting can be used. Continuous casting technology for 13Cr is relatively mature. However, due to the higher alloying element content in 15Cr super martensitic stainless steel, the resulting banded structure is difficult to eliminate during subsequent hot working and heat treatment. Therefore, die casting is currently more suitable. During the smelting process, in addition to maintaining a narrow control over the composition to reduce P, S, the five harmful elements, and gas content, attention must also be paid to controlling deoxidizing elements and the presence of unavoidable impurities in scrap steel, such as V, Ti, Nb, and Al. Approximately 150 ppm of nitrogen can also be present. These elements contribute to higher-than-expected hardness after heat treatment. These elements must be removed as much as possible during the smelting process, while the heat treatment schedule must be adjusted to meet the required strength and toughness balance.

Figure 1 shows the mechanical properties of a test steel containing 0.015% C-1% Cu and 0.10% V and 150 ppm of nitrogen added at different tempering temperatures. Compared to the composition of the 0.015%C-1%Cu test steel, the addition of residual elements such as V and N, which are often retained during electric furnace steel smelting, increased in strength and hardness, while a decrease in plasticity and toughness, under the same heat treatment schedule. This indicates that the addition of N and V significantly increases the hardness and yield strength after tempering, particularly after high-temperature tempering. This increase is primarily due to the solid solution strengthening effect of N and the precipitation strengthening effect of V at temperatures above 525°C. However, the mechanical properties of the V and N-added test steel at different tempering temperatures indicate that the addition of these residual strengthening elements to the test steel can reduce hardness by increasing the tempering temperature. At a tempering temperature of 550-575°C, the hardness can be reduced to below 32 HRC while maintaining the required strength and impact toughness. Therefore, in industrial production processes, on-site heat treatment schedules should be developed based on the actual impurity element levels of the material and on the basis of basic laboratory principles.

4. Seamless Pipe Production and Microstructure, and Property Control

Oil-well pipes are typically formed using a hot piercing and hot rolling process. The billet should be as uniform as possible before rolling to avoid the formation of longitudinal banding after rolling. Homogenization treatment or billet upsetting can be performed after ingot slab formation to improve compositional and microstructure uniformity, without significantly increasing costs. Massive retained austenite, caused by compositional segregation, is a common type of banding in 15Cr super-martensitic stainless steel. This can lead to uneven mechanical properties and, if present in large quantities, can also affect dimensional stability. Generally, a double tempering process can fully eliminate retained austenite, reducing it to a level where it has little impact on mechanical properties. Normalizing followed by high-temperature tempering can almost eliminate it. For seamless pipes, secondary tempering is often used for on-site heat treatment, primarily to eliminate the adverse effects of retained austenite on material properties.

During heat treatment of oil-well pipes, grain size must be controlled. Finer grains are more conducive to achieving both high strength and toughness, while also improving corrosion resistance to a certain extent. Grain size control is primarily achieved through two process steps: dynamic recrystallization during hot working and the normalizing heat treatment regime. High-alloy martensitic stainless steels exhibit strong hardenability and generally do not undergo a γ→α phase transformation during normalizing. Once mixed crystals or coarse grains develop, coarse prior austenite grains are inherited into the final structure along with the martensitic transformation. Therefore, sufficient dynamic recrystallization must occur during hot working to achieve a uniform structure.

When the sample was compressed to 70%, the grain size morphology of the 0.03C-15.5Cr-6.5Ni-1Cu test steel was observed at different temperatures and strain rates. It can be seen that when the deformation temperature is less than 1100°C, a deformation rate of 0.1 s⁻¹ is optimal for achieving a uniform and fine recrystallized structure. When the deformation temperature exceeds 1100°C, complete recrystallization occurs at both 0.1 s⁻¹ and 1 s⁻¹. A deformation rate of 1 s⁻¹ is recommended during this stage to prevent excessive recrystallized grain size. Grain growth is particularly pronounced at temperatures above 1200°C. Hot piercing of seamless pipes involves rapid deformation, making it more appropriate to consider the dynamic recrystallization laws under fast strain rates. Therefore, a hot piercing forming temperature of 1100-1150°C is recommended.

Normalizing heating temperature also significantly affects grain size. Higher normalizing temperatures promote the full solutionization of alloying elements, allowing fine, dispersed precipitation at the martensite lath interfaces during tempering. However, excessively high normalizing temperatures can lead to coarsening of grains. Therefore, the optimal normalizing temperature balances solutionization and grain size. Observation of the grain size of 15Cr super martensitic stainless steel seamless pipes after solution treatment at 950-1040°C reveals that normalizing at temperatures below 980°C results in relatively fine grains, resulting in a rating of 7-8. Normalizing at 1040°C results in significantly larger grains, resulting in a rating of 5. From the perspective of grain control, the normalizing temperature should be kept below 980°C. The final determination of the normalizing temperature also requires comprehensive consideration of the solid solution effect of alloying elements, specifically the influence of the normalizing temperature on the strength-toughness match. Table 2 shows the mechanical properties of 15Cr super martensitic stainless steel seamless pipe at different normalizing temperatures. Normalizing treatments at 950-1040°C show minimal differences in strength and toughness, both meeting the technical requirements of a 125ksi steel grade. This indicates that even with a 950°C normalizing treatment, the material’s solid solution effect is sufficient. Therefore, the normalizing temperature for seamless pipes can be adjusted within the 950-980°C range, ensuring a balanced strength-toughness match and grain size.

5. Current Status and Prospects of Domestic 125ksi Super Martensitic Stainless Steel Production and Application

Currently, 13Cr stainless steel oilfield pipes are still widely used in domestic oil and gas fields. 15Cr material is expected to be used in ultra-deep well sections with downhole temperatures exceeding 180°C in the future. Due to its relatively weak resistance to H₂S corrosion, 15Cr material is more commonly used in gas wells exposed to CO₂ corrosion. Currently, only a small amount of demand exists in some gas wells in the Tarim Oilfield. Due to limited demand, domestic manufacturers lack experience in the mass production of 15Cr super martensitic oilfield pipes. Baosteel Co., Ltd. provided industrial trial products to oilfields, but these products were not adopted for safety reasons. The small amount of 15Cr oilfield pipes currently used in oilfields is produced by JFE. Compared to JFE products, the biggest gap between domestic 15Cr oilfield pipes and JFE products remains in the manufacture and reliability of threaded joints. The main differences in raw materials are reflected in inferior corrosion resistance compared to JFE products and a tendency for their hardness to be higher. These differences in raw materials stem primarily from a lack of in-depth understanding of alloy design principles and the matching of strength, toughness, and corrosion resistance. In recent years, Daye Special Steel has also launched trial production of 15Cr super-martensitic oil pipe, successfully producing seamless pipes measuring Ø88.9mm x 7.34mm. The company also plans to use Tianjin Steel Pipe’s TP-G2 buckle type for specialized threaded joints. The mechanical properties of Daye Special Steel’s seamless pipes are shown in Table 3. A comparison with JFE products of the same specification reveals that Daye Special Steel’s pipes have a comparable strength-toughness balance to JFE’s oil pipes, while maintaining a lower hardness. This demonstrates that through comprehensive control of composition, structure, properties, and processing, the raw materials themselves can reach comparable international standards.

Conlusion

Although current market demand is low, deep wells of 10,000 meters or more are an inevitable development trend for China’s oil and gas field development. With the increasing number of deep and ultra-deep wells, the application of 125ksi steel grade 15Cr super martensitic oil well pipes has a promising future. Currently, developing and accumulating technology and production experience for matching the strength, toughness, and corrosion resistance of 15Cr super martensitic stainless steel, and continuously optimizing the design, manufacturing process, and application reliability of specialized threaded buckles are essential to counter the future monopoly of similar foreign products, enhance the competitiveness of domestic products, and effectively promote domestic materials into high-end engineering applications.