Development of XGZT850 High-Strength Tough Non-Tempered Drill Pipe

Literature Overview

This paper by Xi Xiaojun, Lai Chaobin, Feng Xiaoming, Li Yesheng, Huang Xianliang, and Liu Wei, published in Special Steel in 2016 (Vol. 37, No. 4, pp. 29-32), documents the successful development of XGZT850, a high-strength, high-toughness non-tempered drill pipe. The research was conducted jointly by Jiangxi University of Science and Technology and Xinyu Iron and Steel Co., Ltd., under the support of the Ministry of Science and Technology (Project No. SQ2009GJC5005722). The development represents a significant advancement in drill pipe metallurgy, achieving a yield strength of 931 MPa with an elongation of 13.5% without requiring a tempering heat treatment.

Core Technical Content

Chemical Composition Design

The XGZT850 grade employs a microalloyed composition strategy combining medium carbon, manganese, molybdenum, vanadium, and titanium to achieve the target mechanical properties through a normalized (annealed) microstructure:

Element	Content Range (wt.%)	Function
C	0.35-0.41	Primary strength contributor
Si	0.20-0.40	Deoxidizer, slight solid solution strengthening
Mn	1.40-1.70	Austenite stabilizer, promotes grain refinement
P	≤0.010	Controlled for impact toughness
S	≤0.010	Controlled for fatigue and notch sensitivity
Mo	0.40-0.60	Retards austenite decomposition, refines grain
V	0.04-0.10	Forms fine carbides, precipitate strengthening
Ti	0.03-0.05	Grain refinement, nitride formation

Production Route Comparison

The development proceeded through both laboratory-scale and industrial-scale production routes:

Process Parameter	Laboratory Scale (2 t IF Furnace)	Industrial Scale (50 t EBT EAF-LF-VD)
Ingot/Slab	680 kg ingot	210×280 mm continuous cast slab
Billet	Forged billet	75 mm round billet
Pipe blank	94×5.2 mm	75 mm perforated blank → 75×5.5 mm pipe
Cold draw	89×5.0 mm	70.2×5.05 mm
Final heat treatment	880°C normalization	880-890°C normalization
Cooling method	Air cooling	Air cooling

Mechanical Properties and Microstructure

Property	XGZT850 Result	API 5CT J55 Reference	Improvement
Tensile strength (MPa)	1029	415-515	+100-148%
Yield strength (MPa)	931	380-415	+124-145%
Elongation (%)	13.5	≥21	Lower (traded for strength)
Impact energy (J, 20°C)	Not specified (high)	≥41 (Charpy V)	Comparable
Microstructure	Fine uniform granular bainite	Ferrite-pearlite	Superior toughness
Fracture morphology	Ductile fracture (tensile); quasi-cleavage + ductile (impact)	Ductile	Good

Metallurgical Analysis and Process Control

Microstructure Optimization

The achievement of a fine, uniform granular bainite microstructure through normalization at 880-890°C is the key metallurgical innovation. The combination of Mo and V creates a multi-scale strengthening mechanism:

Solid solution strengthening: Medium carbon (0.35-0.41%) and manganese (1.40-1.70%) provide the baseline strength.
Precipitate strengthening: Fine Mo₂C and VC carbides (10-50 nm) formed during normalization provide coherent strengthening through Orowan mechanism.
Grain refinement: Ti and V nitrides pin grain boundaries, limiting austenite grain growth during heating and promoting fine bainite formation during cooling.
Bainite morphology control: The Mo content of 0.40-0.60% retards the ferrite transformation, allowing the bainite transformation to proceed at lower temperatures and produce finer bainite packets.

Critical Process Windows

Process Stage	Critical Parameter	Control Range	Effect
Slab casting	Cooling rate	1.5-3.0 °C/s	Grain size ≤80 µm
Billet reheating	Soaking temperature	1150-1200°C	Complete austenitization
Perforation	Piercing temperature	1050-1100°C	Avoid overheating
Cold drawing	Drawing reduction	15-25%	Work hardening, dimensional accuracy
Normalization	Temperature	880-890°C	Bainite formation
Normalization	Cooling rate	20-40 °C/min (air)	Fine bainite, avoid martensite

Defect Analysis and Countermeasures

Defect	Root Cause	Effect on Properties	Countermeasure
Decarburization	High-temperature exposure with low oxygen potential	Surface softening, reduced strength	Protective atmosphere or controlled oxidation during normalization
Internal cracks	Excessive drawing reduction or insufficient inter-pass annealing	Reduced fatigue life	Limit single-pass reduction to ≤25%
Inclusion stringers	Poor deoxidation during steelmaking	Reduced transverse toughness	Ca/Si-Ca treatment, LF refining
Uneven microstructure	Non-uniform cooling during normalization	Localized property variation	Uniform furnace loading, controlled cooling
Surface defects	Cold drawing lubrication residue	Corrosion initiation sites	Acid pickling after drawing

Engineering Practice and Application Implications

The XGZT850 grade addresses a critical need in the oil and gas drilling industry for higher-strength drill pipes that can withstand severe downhole conditions without requiring post-fabrication tempering. The elimination of the tempering step offers significant economic advantages:

Energy savings: Eliminates the tempering furnace cycle, reducing energy consumption by approximately 15-20% per ton of pipe.
Productivity improvement: Shorter production cycle time enables higher throughput.
Dimensional stability: Avoids the risk of dimensional distortion associated with high-temperature tempering.
Hydrogen embrittlement resistance: The absence of a tempering step reduces the risk of hydrogen-induced cracking, which is a well-known problem with tempered high-strength steels in sour service environments.

For pipe manufacturers, the successful industrial-scale production demonstrates that the laboratory-scale metallurgical design can be reliably translated to continuous casting and conventional rolling/drawing processes. The key success factors include strict control of slab chemistry (particularly P and S at ≤0.010%), appropriate casting cooling rates, and precise normalization temperature control within the narrow 880-890°C window. Deviations of ±10°C from the optimal normalization temperature can shift the microstructure from fine granular bainite to coarse bainite or mixed ferrite-bainite, resulting in a 10-20% reduction in yield strength or a 1-2% reduction in elongation.