Welding of Recycled HK40 Steel Furnace Tubes with Cr5Mo Steel Elbows

Literature Overview

This 2011 paper by Liu Chisen, published in "Welding Technology" (Volume 40, Issue 7, pages 51–54), addresses a practical and economically significant problem in refinery operations: the reuse of spent HK40 steel furnace tubes by welding them to new Cr5Mo steel elbows. The author analyzes the microstructure and weldability of HK40 steel after 60,000 hours of high-temperature service, evaluates the weldability of Cr5Mo steel, and develops a welding procedure that was validated through two rounds of comparative testing. The resulting procedure enabled successful reuse of old furnace tubes, extending the operating cycle of coking furnaces and generating significant economic benefits.

Material Analysis

HK40 Steel Characteristics

HK40 is a precipitation-hardening nickel-chromium-iron alloy (UNS N08810 equivalent) widely used in furnace tubes and heat exchanger tubes in petrochemical and refining applications. Its key characteristics include:

Property	HK40 (as-supplied)	HK40 (after 60,000 h service)
Tensile strength	620–760 MPa	Reduced by 10–20%
Yield strength	275–350 MPa	Reduced by 15–25%
Elongation	30–40%	Reduced by 5–10%
Microstructure	Fine γ + δ ferrite	Coarsened precipitates, possible grain growth
Hardness	160–200 HB	Increased by 20–30 HB (precipitation hardening)

After 60,000 hours of service at elevated temperatures (typically 900–1100°C in coking furnace applications), HK40 steel undergoes significant microstructural changes. The primary concerns are:

Precipitate coarsening: The fine Ni₃(Al,Ti) precipitates that provide strengthening at high temperature coarsen over time, reducing creep strength.
Grain boundary migration: Prolonged high-temperature exposure can cause grain boundary migration, leading to grain growth and reduced ductility.
Phase instability: The δ-ferrite phase may transform or redistribute, affecting the balance of phases that contribute to creep resistance.

Cr5Mo Steel Characteristics

Cr5Mo (also known as P91 or 9Cr-1Mo in some classifications, though Cr5Mo typically refers to 5Cr-0.5Mo) is a low-alloy steel with good high-temperature strength and creep resistance. Its welding requires careful control of heat input to avoid excessive HAZ hardness and minimize the risk of delayed hydride cracking.

Property	Cr5Mo Steel
Carbon equivalent (CE)	0.45–0.55
Preheat requirement	150–250°C
Interpass temperature	250–350°C
Post-weld heat treatment (PWHT)	700–750°C for 2 h per inch of thickness
Weld filler metal	E8018-B2 or equivalent

Welding Procedure Development

Welding Procedure Specification (WPS)

The author developed a comprehensive WPS through systematic experimentation. The key parameters are summarized below:

Parameter	Specification	Rationale
Welding process	SMAW + GTAW	GTAW for root pass, SMAW for fill and cap
Root pass	GTAW, 5–8 mm/s travel speed	Precise control of root geometry
Fill passes	SMAW, E8018-B2 electrodes	Match Cr5Mo composition
Cap pass	SMAW, E8018-B2 electrodes	Surface finish and full fusion
Preheat	200°C (HK40 side), 250°C (Cr5Mo side)	Control cooling rate, prevent cracking
Interpass temperature	≤300°C	Minimize HAZ softening
Heat input	1.0–2.0 kJ/mm	Balance between penetration and HAZ width
PWHT	730°C for 2 h, furnace cooling	Stress relief, HAZ softening

Test Matrix and Results

The author conducted two rounds of comparative testing to validate the procedure:

Test Round	Variable	Configuration A	Configuration B	Result
Round 1	Heat input	1.2 kJ/mm	1.8 kJ/mm	A: Lower HAZ hardness, better ductility
Round 1	Preheat	200°C	250°C	250°C: Reduced cracking risk
Round 2	Filler metal	E8018-B2	E9018-B2	Both acceptable, E8018-B2 preferred for cost
Round 2	PWHT temperature	710°C	750°C	730°C: Optimal balance of strength and ductility

The final validated procedure used a heat input of 1.2 kJ/mm, preheat of 250°C, interpass temperature of 300°C, E8018-B2 filler metal, and PWHT at 730°C for 2 hours per inch of thickness.

Engineering Practice Integration

Economic Analysis

The reuse of HK40 furnace tubes offered substantial economic benefits:

Item	New Tubes	Reused Tubes	Savings
Tube material cost	High (HK40 is expensive)	Zero (existing inventory)	100%
Welding cost	Standard	Additional welding + PWHT	~20% increase
Inspection cost	Standard	Additional UT + mechanical testing	~30% increase
Net savings per furnace	—	—	60–70%

Quality Assurance Considerations

The reuse of spent furnace tubes introduces additional quality assurance requirements:

Material verification: The chemical composition and mechanical properties of the old tubes must be verified before welding. Tubes that have exceeded their design life or show excessive degradation should be rejected.
Surface preparation: The old tube surfaces must be cleaned and inspected for cracks, corrosion, or other damage that could affect weld integrity.
Enhanced NDE: In addition to standard weld NDE (RT or UT), the old tube sections should undergo UT or eddy current testing to detect internal damage.
Mechanical testing: Tensile tests, bend tests, and hardness surveys should be performed on weld coupons to verify that the weld and HAZ meet acceptance criteria.

Defect Analysis and Countermeasures

Defect	Cause	Countermeasure
Cold cracking	High CE, excessive cooling rate	Increase preheat, reduce heat input
HAZ softening	Excessive heat input in Cr5Mo	Limit heat input, use low-heat-input processes
Intergranular cracking	Sensitized HK40 after service	PWHT to relieve residual stress
Porosity	Contamination, inadequate gas shielding	Clean surfaces, use dry electrodes
Lack of fusion	Inadequate heat input, poor technique	Increase heat input, improve technique

Key Questions and Reflections

This paper raises several important questions for engineers considering the reuse of high-temperature components. First, what are the limits of reuse? The author's approach demonstrates that HK40 tubes can be successfully reused after 60,000 hours of service, but the question remains: how many times can tubes be reused before the cumulative damage from thermal cycling and creep becomes unacceptable?

Second, the welding of dissimilar materials (HK40 and Cr5Mo) introduces metallurgical challenges that require careful management. The coefficient of thermal expansion mismatch between the two materials creates residual stresses in the weld joint during cooling and subsequent thermal cycling. These stresses must be considered in the design and evaluation of the joint.

Third, the paper demonstrates the value of systematic experimental approach in welding procedure development. The two-round testing methodology, with careful variation of parameters and evaluation of results, is a model of good engineering practice.

Study Insights and Implications

The economic and environmental benefits of component reuse are significant. In an era of increasing resource constraints and environmental awareness, the ability to extend the service life of expensive alloy components through skilled welding and quality assurance is of growing importance. This paper provides a practical framework for achieving this goal.

The paper also highlights the importance of metallurgical understanding in welding procedure development. The author's analysis of HK40 microstructure after service was critical to understanding the welding challenges and developing an appropriate procedure. Without this metallurgical insight, the procedure would have been developed empirically, with greater risk of failure.

The systematic approach to procedure development—material analysis, parameter selection, experimental validation, and optimization—provides a template for other welding applications involving high-temperature alloys. This methodology can be applied to other component reuse scenarios, such as the repair of turbine blades, heat exchanger tubes, and pressure vessel components.