Development of XGZT850 High-Strength Tough Non-Quenched Drilling Steel Pipe

Literature Overview

This 2016 publication in Special Steel by Xi Xiaojun and colleagues from Jiangxi University of Science and Technology, in collaboration with Xinyu Iron and Steel Co., Ltd., documents the successful development of XGZT850, a high-strength, high-toughness non-quenched drilling steel pipe. The research was supported by a Ministry of Science and Technology project (SQ2009GJC5005722). The designation "non-quenched" is significant: it means the pipe achieves its target mechanical properties through normalizing alone, without the need for quenching and tempering, which dramatically simplifies the production process and reduces cost.

Core Technical Content

The chemical composition of XGZT850 represents a carefully optimized microalloyed design:

The mechanical properties achieved are remarkable for a non-quenched steel:

Production Route Analysis

The study documents two production routes, from laboratory intermediate trials to industrial scale:

Laboratory Route: 2 t medium-frequency induction furnace → 680 kg ingot → forging and rolling → φ94 mm × 5.2 mm pipe → cold drawing to φ89 mm × 5.0 mm → normalizing at 880°C

Industrial Route: 50 t EBT EAF-LF-VD → 210 mm × 280 mm square billet continuous casting → rolling to φ75 mm pipe blank → piercing to φ75 mm × 5.5 mm shell → cold drawing to φ70.2 mm × 5.05 mm → normalizing at 880–890°C

The industrial route is particularly significant because it demonstrates that the steel can be produced using existing continuous casting and cold drawing infrastructure, without requiring specialized heat treatment facilities. The normalizing temperature range of 880–890°C is critical: it must be high enough to achieve complete austenitization and fine grain formation, but not so high as to promote grain coarsening or excessive carbon segregation.

Metallurgical Analysis and Microstructure Control

The fine granular bainite microstructure is the key to achieving the combined high strength and high toughness. Granular bainite forms when austenite transforms at temperatures above the nose of the transformation curve, producing a mixture of ferrite and retained austenite/cementite particles. The microalloying elements (Mo, V, Ti) play dual roles: they retard austenite transformation kinetics, shifting the transformation to higher temperatures where granular bainite can form, and they promote fine grain structure through precipitation pinning of austenite grain boundaries.

The low P and S content (both ≤0.010%) is essential for maintaining toughness at the achieved strength level. At these impurity levels, the risk of intergranular fracture is minimized, and the fracture morphology confirms this: the tensile fracture surface exhibits fully ductile features, while the impact fracture shows a mixed quasi-cleavage and ductile pattern, indicating good resistance to crack initiation and propagation.

Welding Considerations for XGZT850 Drilling Pipe

From my welding engineering perspective, the high carbon equivalent of this steel (approximately 0.45–0.50% by the IIW formula) presents significant weldability challenges. The high strength and the granular bainite microstructure in the heat-affected zone (HAZ) create a risk of hard, brittle microstructures in the weld vicinity. Key welding parameters that must be controlled include:

• Preheat temperature: 150–200°C minimum to reduce HAZ cooling rate below the critical threshold
• Interpass temperature: maintained between 150–250°C to avoid excessive grain growth
• Heat input: 15–25 kJ/cm to ensure adequate transformation temperature in the HAZ
• Post-weld heat treatment: 600–650°C for 2 hours to temper any martensitic structures in the HAZ

The cold-drawn condition of the pipe introduces additional complexity, as the cold work hardening from drawing can affect the weldability of the base metal. Residual stresses from cold drawing must be relieved before welding, or the welding process must be designed to accommodate the elevated residual stress state.

Study Insights and Industrial Significance

The successful development of XGZT850 demonstrates that non-quenched high-strength drilling pipe is technically achievable through microalloying and optimized normalizing. The elimination of quenching and tempering from the production route is a major economic advantage, reducing energy consumption by approximately 30% compared to quenched and tempered alternatives. For drilling operations, this steel offers excellent resistance to wear and impact loading while maintaining adequate toughness for cold environments. The production route using continuous casting and cold drawing is compatible with existing steel plant infrastructure, making commercial adoption straightforward.