Research and Practice of Large-Diameter Austenitic Stainless Steel Seamless Pipes

Literature Overview

The paper by Chen Xiang from Zhejiang Glos Seamless Steel Pipe Co., Ltd., published in "Steel Pipe" (Vol. 43, No. 5, 2014), documents the production technology and quality control measures for large-diameter austenitic stainless steel seamless pipes of grades TP304, TP316, TP321, and TP347. The work covers the complete production process from billet preparation through hot rolling and heat treatment, with emphasis on process control parameters that ensure acceptable product quality.

Production Process Flow

The production of large-diameter austenitic stainless steel seamless pipes follows a conventional hot-rolling route adapted for the specific metallurgical requirements of 300-series stainless steels:

Process Steps and Key Parameters

Process Step	Key Parameter	Control Range	Purpose
Billet heating	Temperature	1150–1250 °C	Achieve uniform temperature and appropriate grain size
Billet grain size	ASTM grain size	5–7	Control plasticity and hot workability
Conical roll piercing	Piercing temperature	≥1100 °C	Form hollow shell without excessive deformation
Mandrel drawing	Drawing reduction	10–25% per pass	Reduce wall thickness uniformly
Reduction rolling	Rolling temperature	950–1100 °C	Achieve dimensional accuracy
Finishing rolling	Final temperature	≥900 °C	Maintain austenitic structure
Heat treatment	Solution temperature	1050–1150 °C	Dissolve carbides and restore corrosion resistance
Quenching	Cooling medium	Water or air	Rapid cooling to prevent carbide precipitation

Key Technical Controls

Billet Preparation and Heating

The grain size of the austenitic stainless steel billet is a critical factor that influences the entire downstream processing. Excessive grain growth during heating leads to poor hot workability, surface cracking during rolling, and coarse microstructure in the final product. The authors emphasize controlling the heating rate to 2–3 °C/min for the initial stage and maintaining a holding time of 2–4 hours at the target temperature.

The chemical composition of the 300-series grades introduces specific challenges. The high chromium and nickel content increases the hot cracking susceptibility during piercing and rolling. The addition of titanium (TP321) or niobium (TP347) as stabilizers helps prevent chromium carbide precipitation at grain boundaries, but these elements can form low-melting-point phases that affect hot workability.

Hot Rolling Process Control

The use of a conical roll piercing mill combined with a reduction mill equipped with rolling angle rolls is highlighted as a feasible approach for large-diameter production. The conical roll piercing provides better control over the initial hollow shell geometry, while the rolling angle configuration on the reduction mill ensures more uniform wall thickness distribution.

Key rolling parameters include:

Total reduction ratio: 2.5–3.5 (billet to pipe)
Number of rolling passes: 4–6
Rolling speed: 0.5–1.5 m/s
Inter-pass temperature maintenance: ≥950 °C throughout the rolling sequence

Heat Treatment

The solution heat treatment is the most critical post-rolling process for austenitic stainless steel pipes. The treatment must dissolve all chromium carbides (Cr₂₃C₆, Cr₇C₃) that may have precipitated during rolling or subsequent cooling. Incomplete dissolution results in chromium-depleted zones adjacent to grain boundaries, leading to intergranular corrosion susceptibility.

Grade	Solution Treatment Temperature	Quenching Medium	Minimum Hold Time
TP304	1050–1100 °C	Water	10 min per 25 mm thickness
TP316	1050–1100 °C	Water	10 min per 25 mm thickness
TP321	1050–1150 °C	Water or air	15 min per 25 mm thickness
TP347	1050–1150 °C	Water or air	15 min per 25 mm thickness

Quality Verification

The product performance verification results confirm that the production route is technically feasible. Typical quality indicators include:

Tensile strength: ≥515 MPa (TP304), ≥515 MPa (TP316), ≥515 MPa (TP321/TP347)
Yield strength: ≥205 MPa
Elongation: ≥40%
Impact energy (Charpy V-notch, 20 °C): ≥47 J
Grain size: ≥5 (ASTM)
Intergranular corrosion: Pass (ASTM A262 Practice E)
Surface quality: No cracks, laps, or rolling defects

Study Insights and Engineering Practice

The paper demonstrates that large-diameter austenitic stainless steel seamless pipes can be successfully produced using conventional hot-rolling equipment when process parameters are carefully controlled. The emphasis on billet grain size control and rolling temperature maintenance reflects the well-known sensitivity of austenitic stainless steels to thermal processing.

From an engineering practice perspective, I would highlight several additional considerations that are not explicitly addressed in the paper. First, the cooling rate after solution heat treatment significantly affects the final microstructure. Water quenching is preferred for thicker sections to ensure rapid cooling through the sensitization temperature range (450–850 °C), while air cooling may be acceptable for thinner walls. Second, the internal surface quality of the pipe is often overlooked but is critical for applications in chemical processing and pharmaceutical industries where internal corrosion resistance is paramount. Third, the dimensional tolerance of large-diameter pipes must account for the greater tendency toward ovality during rolling, which requires careful adjustment of the reduction mill roll configuration.

Reference Value and Outlook

This work provides practical guidance for mills considering entry into the large-diameter austenitic stainless steel seamless pipe market. The process parameters documented here represent a proven production approach that can serve as a baseline for new product development. Future improvements could include the use of thermomechanical controlled processing (TMCP) to achieve finer grain structures without requiring excessive solution treatment temperatures, and the application of online metallographic analysis to provide real-time feedback on microstructural evolution during rolling.