Cold and Hot Cracking Resistance Analysis of Pressure Steel Pipe Welding Joints

Literature Overview

The paper by Chen Guoxin, Lei Xianming, Zeng Zhouliang, and Chen Zhigang, published in Hot Working Technology (2006, Vol. 35, No. 7, pp. 29-30), presents a systematic analysis of the cold and hot cracking susceptibility of welding joints in pressure steel pipes fabricated from NK-HITEN 610U2 steel plate. The authors, from the Department of Mechanical and Energy Engineering at Shaoyang University, evaluated the welding crack tendency from three perspectives: chemical composition of the base material, diffusable hydrogen content in the weld metal, and structural restraint degree. Based on this evaluation, they proposed corresponding process measures to mitigate cracking risks.

Core Technical Analysis

Welding-induced cracking in pressure steel pipes is a critical quality issue that directly affects the structural integrity and service life of pressure-containing equipment. The NK-HITEN 610U2 steel, a high-strength low-alloy (HSLA) steel with a yield strength of approximately 610 MPa, is commonly used in pressure vessels and pipelines due to its favorable combination of strength and toughness. However, its high strength also makes it susceptible to both cold cracking (hydrogen-induced cracking) and hot cracking (solidification cracking) during welding.

Cold Cracking Analysis

Cold cracking, also known as delayed cracking, typically occurs in the heat-affected zone (HAZ) of high-strength steels after welding, often hours or even days after the welding operation. The three factors governing cold cracking susceptibility are well-established in welding metallurgy:

Factor	Assessment for NK-HITEN 610U2	Critical Threshold
Carbon Equivalent (CE)	CE = C + Mn/6 + (Cr+Mo+V)/5 + (Ni+Cu)/15 ≈ 0.45-0.55%	CE > 0.45% indicates significant cold cracking risk
Diffusable Hydrogen Content	Typically 2-5 mL/100g in as-welded condition	Should be controlled below 5 mL/100g for HSLA steels
Structural Restraint Degree	High restraint in thick-walled pressure pipe sections	Restraint stress > 0.3σ_y increases crack risk

The authors emphasize that the carbon equivalent of NK-HITEN 610U2 is relatively high, which means the steel has limited weldability without proper preheating and post-weld treatment. The CE value, calculated using the IIW formula, serves as a primary indicator of cold cracking susceptibility. When CE exceeds 0.45%, the risk of martensite formation in the HAZ increases significantly, creating a hard and brittle microstructure that is prone to hydrogen-induced cracking.

Hot Cracking Analysis

Hot cracking occurs during solidification or at elevated temperatures in the weld metal. The susceptibility to hot cracking depends on the weld metal composition, particularly the sulfur and phosphorus content, and the solidification mode. For NK-HITEN 610U2, the base material typically has low sulfur content (< 0.02%), which reduces the risk of sulfide-type hot cracking. However, the use of inappropriate filler metals with high sulfur or phosphorus content can introduce hot cracking susceptibility.

The authors propose the following process measures to mitigate both cold and hot cracking:

Preheating: A preheat temperature of 100-150°C is recommended to reduce the cooling rate in the HAZ, minimize martensite formation, and allow diffusable hydrogen to escape.
Interpass temperature control: Maintaining interpass temperature between 150-250°C to prevent excessive cooling rates in multi-pass welds.
Post-weld heat treatment (PWHT): A stress-relief annealing at 550-650°C for 1-2 hours per 25 mm of thickness to relieve residual stresses and further reduce hydrogen content.
Filler metal selection: Using low-hydrogen filler metals (E7018 or equivalent) with diffusable hydrogen content below 5 mL/100g.
Welding sequence optimization: Adopting a welding sequence that minimizes restraint stress concentration, such as welding from the center outward or using back-step welding.

Process Parameter Recommendations

Process Parameter	Recommended Range	Rationale
Preheat temperature	100-150°C	Reduce HAZ hardness, minimize martensite
Interpass temperature	150-250°C	Control cooling rate in multi-pass welds
Heat input	10-25 kJ/mm	Balance cooling rate and HAZ width
Diffusable hydrogen	< 5 mL/100g	Below critical threshold for HSLA steels
PWHT temperature	550-650°C	Stress relief without excessive grain growth
PWHT duration	1-2 h per 25 mm thickness	Ensure uniform temperature throughout section

Engineering Practice Integration

In engineering practice, the welding of pressure steel pipes requires strict adherence to welding procedure specifications (WPS) that incorporate the findings of such cracking analyses. The welding procedure qualification record (PQR) must demonstrate that the selected parameters produce a weld with acceptable mechanical properties and no cracking defects. Non-destructive testing (NDT) methods such as magnetic particle testing (MT) and liquid penetrant testing (PT) are essential for detecting surface and near-surface cold cracks, while ultrasonic testing (UT) can identify subsurface hot cracks.

The analysis presented in this paper follows a systematic approach that aligns with the FMEA (Failure Mode and Effects Analysis) methodology commonly used in quality management. By identifying the failure modes (cold cracking and hot cracking), analyzing their causes (chemical composition, hydrogen, restraint), and proposing preventive measures (preheating, PWHT, filler selection), the authors provide a structured framework that can be directly applied in engineering practice.

Study Insights and Implications

This literature underscores the critical importance of a systematic approach to welding crack prevention in high-strength steel applications. The three-factor analysis of cold cracking (CE, hydrogen, restraint) remains the gold standard in welding metallurgy, and the proposed process measures are well-aligned with industry standards such as AWS D1.1 and EN ISO 15614. For engineers working with NK-HITEN 610U2 or similar HSLA steels, this paper provides a practical reference for welding procedure development. It is also worth noting that the analysis could be further enhanced by incorporating thermal modeling to predict the actual cooling rates at different locations in the weldment, which would provide more precise guidance for preheat and interpass temperature selection. The work serves as a valuable reminder that welding quality in pressure applications is not merely a matter of following standard procedures but requires a deep understanding of the metallurgical interactions at play.