Welding of Reused HK40 Steel Furnace Tubes with Cr5Mo Steel Elbows

Literature Overview

This paper by Liu Chisen, published in 2011 in the journal "Welding Technology" (Volume 40, Issue 7), addresses a practical engineering challenge in refinery operations: the welding of previously serviced HK40 steel furnace tubes to Cr5Mo steel elbows. The component has accumulated 60,000 hours of high-temperature service, during which significant microstructural changes have occurred. The paper describes the metallurgical analysis of the aged HK40 steel, the welding performance evaluation of both materials, and the development of a welding procedure that enables the successful reuse of the old furnace tubes, thereby extending the operating cycle of the coking furnace and achieving significant economic savings.

Metallurgical Analysis of Aged HK40 Steel

HK40 is a nickel-chromium-iron superalloy commonly used in high-temperature furnace applications due to its excellent creep resistance and oxidation resistance at temperatures up to approximately 1100 degrees Celsius. After 60,000 hours of service in a coking furnace, the microstructure undergoes significant changes that directly impact weldability:

Microstructural Feature	As-Supplied Condition	After 60,000 h Service	Impact on Weldability
Grain boundary carbides	Fine, dispersed	Coarsened, spheroidized	Reduced hot cracking resistance
Precipitate phase (gamma-prime)	Uniform distribution	Coarsening and coalescence	Lower local ductility
Oxidation scale	Minimal	Significant external scale	Contamination risk at weld
Grain size	Controlled	Possible grain growth	Reduced creep resistance in HAZ
Carbon distribution	Homogeneous	Segregation at grain boundaries	Hot cracking susceptibility

The coarsening of precipitates and grain boundary carbides after prolonged service creates a material that is more susceptible to hot cracking during welding. The oxidation scale on the surface, if not properly removed, can introduce oxygen into the weld metal, leading to porosity and reduced mechanical properties.

Cr5Mo Steel Welding Characteristics

Cr5Mo steel (equivalent to P91 in some designations, though P91 has higher Cr and Mo content) is a chromium-molybdenum alloy steel widely used in power generation and refinery applications. Its welding characteristics include:

Preheat requirement: 200-300 degrees Celsius to control cooling rate and prevent hydrogen-induced cracking
Interpass temperature: Must be maintained below 300 degrees Celsius to avoid over-aging
Post-weld heat treatment (PWHT): Required at 720-760 degrees Celsius for 2-4 hours to relieve residual stresses and promote tempering
Welding consumables: Low hydrogen electrodes or flux-cored wires with matching or slightly higher alloy content
Hydrogen control: Electrodes baked at 350-400 degrees Celsius, strict moisture control

Welding Procedure Development

The welding procedure was developed through two rounds of comparative testing, which is a systematic approach consistent with the PDCA (Plan-Do-Check-Act) methodology:

Round 1 Testing:

Evaluated base metal weldability through coupon tests
Identified critical welding parameters: preheat temperature, interpass temperature, and post-weld treatment
Tested multiple consumable options for compatibility with both materials

Round 2 Testing:

Refined parameters based on Round 1 results
Conducted full-scale weld trials on representative joints
Performed comprehensive NDT and mechanical testing on test welds
Finalized the welding procedure specification (WPS)

The finalized welding procedure included the following key parameters:

Parameter	Specification	Rationale
Welding process	SMAW (SMAW/GTAW combination)	Flexibility for field conditions
Preheat temperature	250-300 degrees Celsius	Control cooling rate, prevent HIC
Interpass temperature	Below 300 degrees Celsius	Prevent over-aging of Cr5Mo
Electrode type	Low hydrogen, matching Cr5Mo	Hydrogen control, alloy matching
Post-weld heat treatment	740 degrees Celsius, 3 hours	Stress relief, tempering
Surface preparation	Grind to bright metal	Remove oxidation scale
NDT method	100% UT + RT	Full coverage of critical weld

Defect Analysis and Countermeasures

Based on the metallurgical analysis and welding trials, the following defect risks were identified and addressed:

Defect Type	Cause	Detection Method	Countermeasure
Hot cracks	Aged HK40 with coarsened carbides	RT or PT	Higher preheat, proper filler selection
Cold cracks	Hydrogen in Cr5Mo HAZ	MT or PT after PWHT	Low hydrogen electrodes, bake electrodes
Porosity	Oxidation scale contamination	RT or UT	Thorough surface cleaning before welding
Lack of fusion	Inadequate heat input	UT	Increase heat input, proper technique
Undercut	Excessive travel speed	Visual or MT	Reduce travel speed, proper electrode angle

Engineering Practice and Economic Impact

The economic case for reusing HK40 furnace tubes rather than replacing them with new material is substantial. The paper reports that this approach:

Saved the cost of new furnace tubes, which represent a significant capital expenditure
Extended the operating cycle of the coking furnace beyond its originally planned maintenance interval
Reduced downtime associated with tube replacement
Demonstrated that properly evaluated and prepared aged components can be safely reintegrated into service

This approach aligns with modern asset management principles that emphasize condition-based assessment over time-based replacement. The systematic metallurgical evaluation of the aged material, followed by procedure qualification through testing, provides the technical basis for the engineering decision to reuse the components.

Study Insights and Reflections

This paper exemplifies the practical application of metallurgical knowledge to solve real engineering problems with significant economic implications. The author's approach of first understanding the microstructural changes in the aged material, then developing a welding procedure that accounts for those changes, represents sound engineering methodology. The two-round testing approach demonstrates the iterative nature of welding procedure development, where initial assumptions are tested, results are analyzed, and parameters are refined based on empirical evidence.

The paper also highlights an important principle in materials engineering: the as-received condition of a material is not always the same as the condition at the time of welding. For components that have undergone prolonged high-temperature service, the microstructure has evolved, and welding procedures developed for new material may not be appropriate. This insight is directly applicable to maintenance welding in power plants, refineries, and chemical plants, where components are routinely welded after extended service periods.