Research on Producing 13Cr Thick-Wall Steel Pipes Using Continuous Casting Billets Instead of Forged Billets

Literature Overview

This technical paper by Gao Jie, Wang Jinyong, Wang Lianhua, Cao Hongbo, Gao Zhenglei, and Liu Fuqiang from Handan Xinxing Special Tube Co., Ltd. was published in Hot Working Technology (Vol. 50, Issue 21, 2021, pp. 98-101). The research investigates the feasibility of using continuous casting (CC) billets as feedstock for hot extrusion production of 13Cr thick-wall seamless steel pipes, comparing the results with conventional forged billet production.

Technical Background and Motivation

13Cr martensitic stainless steel pipes are critical components in oil and gas production systems, particularly for sour service environments where hydrogen-induced cracking (HIC) and sulfide stress cracking (SSC) resistance are required. The thick-wall specification of φ219 mm × 30 mm represents a challenging product that demands high material quality and uniformity.

Traditionally, forged billets have been used for such thick-wall pipe production due to their superior internal quality and uniform microstructure. However, forged billets are significantly more expensive than continuous casting billets. This research investigates whether CC billets can be substituted without compromising the final pipe quality.

Microstructural Analysis

The initial microstructural comparison reveals significant differences between the two billet types:

After 84.5% hot extrusion deformation, both billet types achieved uniform and dense microstructures in the final pipe product. The grain size results demonstrate:

• CC billet pipe: Average grain size 6.5-7.5 grade
• Forged billet pipe: Average grain size 7.0-8.0 grade

The slightly finer grain size in forged billet pipes (0.5-1.5 grade difference) is within acceptable limits for most 13Cr pipe applications. The high deformation ratio of 84.5% was critical in breaking down the coarse columnar grain structure of CC billets and achieving uniform equiaxed grain structures.

Mechanical Properties and δ-Ferrite Analysis

The mechanical properties of pipes produced from both billet types were found to be comparable. This is a critical finding that validates the technical feasibility of CC billet substitution. The δ-ferrite content, which is an important parameter for 13Cr martensitic stainless steels as it affects toughness and corrosion resistance, was also within acceptable ranges for both production routes.

Key Technical Insights

The research demonstrates that the high hot extrusion deformation ratio of 84.5% is the critical factor enabling CC billet substitution. This level of deformation is sufficient to:

1. Break down the coarse columnar grain structure inherited from solidification
2. Eliminate internal porosity and segregation in CC billets
3. Achieve dynamic recrystallization and uniform grain refinement
4. Homogenize the chemical composition through extensive plastic flow

Engineering Practice Recommendations

For manufacturers considering CC billet substitution for thick-wall 13Cr pipe production, the following recommendations are derived from this research:

1. Hot extrusion reduction must be maintained at or above 80% to ensure adequate microstructural refinement.
2. CC billet quality must be controlled to minimize center porosity and macrosegregation.
3. The quenching and tempering heat treatment parameters (980°C × 130 min quench, 720°C × 200 min temper) must be precisely controlled to achieve the target martensitic microstructure with appropriate retained austenite and δ-ferrite content.
4. Non-destructive testing protocols should be maintained at the same rigor as for forged billet production, with particular attention to internal quality verification.

Study Insights and Economic Implications

This research has significant economic implications for the specialty steel pipe industry. The substitution of CC billets for forged billets can reduce raw material costs substantially, potentially by 30-50% depending on market conditions. This cost reduction can be passed to end users or used to improve profit margins, making 13Cr pipe more accessible for sour service applications.

From a metallurgical perspective, the key insight is that sufficient plastic deformation during hot working can overcome the inherent quality limitations of CC billets. The columnar grain structure and internal defects characteristic of CC billets are effectively eliminated through the high reduction ratio extrusion process. This finding has broader implications for other alloy steel pipe applications where CC billet substitution could provide similar economic benefits.

However, engineers should note that this substitution is validated specifically for the 84.5% reduction ratio and the particular heat treatment parameters studied. For different pipe specifications or steel grades, the minimum required deformation ratio and optimal heat treatment parameters may differ. Each substitution should be validated through comprehensive testing including mechanical properties, corrosion resistance, and long-term service performance evaluation.