Field Heat Treatment of 10CrMo910 Large Diameter Welded Tee Using Infrared Heating

Literature Overview

This paper, published in Electric Power Construction (1995, Vol. 16, No. 12) by Fu Jianhua of Fujian Provincial Electric Power Construction Company No. 1, addresses a critical practical challenge in thermal power plant piping construction: the post-weld heat treatment (PWHT) of a 10CrMo910 Φ273×36 single-rib reinforced equal-diameter welded tee. The material 10CrMo910 is a low-alloy martensitic steel widely used in high-temperature steam piping, and the combination of large diameter (273 mm), thick wall (36 mm), and reinforced geometry creates significant challenges for achieving uniform heat treatment in the field. The author describes a hybrid heating approach combining infrared rope heaters with crawler-type (tracked) heaters to achieve controlled, uniform heating of both the internal and external surfaces of the fitting simultaneously.

Technical Background and Material Considerations

10CrMo910 (equivalent to ASTM A335 P91 or EN 10216-2 10CrMo9-10) is a 9Cr-1Mo martensitic steel with a nominal composition of approximately 0.85–1.05% C, 8.5–9.5% Cr, 0.85–1.05% Mo, and ≤0.05% C. This alloy is normalized and tempered to achieve a tempered martensite microstructure with excellent creep resistance at temperatures up to 600°C, making it ideal for supercritical and ultra-supercritical boiler main steam piping.

The wall thickness of 36 mm is significant from a heat treatment perspective. According to ASME B31.1 and GB/T 20801.5, post-weld heat treatment is mandatory for P91-class materials with wall thickness exceeding 19 mm (3/4 inch) to relieve welding residual stresses and temper the heat-affected zone (HAZ). The single-rib reinforcement further complicates the geometry, creating localized stress concentrations at the rib-to-pipe junction that demand careful thermal management.

Heating Method Analysis

The key innovation described in this paper is the combination of two heating technologies to overcome the limitations of either method used alone.

Infrared Rope Heater

Infrared rope heaters operate on the principle of radiative heat transfer, with heating elements embedded in flexible cable sheathed with ceramic or quartz envelopes. Advantages include:

• High heat flux density at the target surface
• Flexible conformability to complex geometries
• Relatively rapid heating rates
• Ability to be wrapped around internal and external surfaces simultaneously

However, infrared rope heaters alone suffer from non-uniform temperature distribution, particularly across thick sections where thermal gradients can be substantial. The 36 mm wall thickness means the temperature difference between the heated surface and the centerline can reach 100–150°C during rapid heating.

Crawler-Type (Tracked) Heater

Crawler-type heaters are designed to travel along the circumference or length of a pipe or fitting, providing distributed convective and radiative heating. Their advantages include:

• More uniform temperature distribution along the heated zone
• Lower peak heat flux, reducing thermal stress
• Suitable for large diameter components where rope heaters alone cannot maintain uniformity

Combined Approach

The combination strategy described achieves:

• Infrared ropes provide baseline radiative heating on internal surfaces and concentrated heating at critical weld zones
• Crawler heaters provide supplementary uniform heating on external surfaces and large planar areas
• Multi-channel independent temperature control allows differential power adjustment to compensate for geometric asymmetries

Heat Treatment Cycle Design

For P91-class materials, the PWHT cycle typically follows these parameters:

The paper emphasizes the importance of maintaining a temperature uniformity of ±25°C across the heated zone during the soaking period. For a 36 mm wall section, this requires careful calibration of heater power distribution and real-time thermocouple monitoring at multiple points (typically minimum 6 thermocouples for a tee of this size, placed at internal and external surfaces at 120° intervals).

Engineering Practice Insights

From my experience with P91 piping heat treatment, several critical lessons emerge from this approach:

1. Thermocouple placement is paramount. For thick-walled components, surface thermocouples alone are insufficient. Embedded thermocouples or calculations based on surface readings must account for thermal lag in the 36 mm section. The centerline temperature typically lags the surface by 15–25 minutes during heating and by a similar margin during cooling.
2. The single-rib geometry creates thermal asymmetry. The rib acts as a thermal mass that absorbs heat differently than the cylindrical shell. Heater power must be increased on the rib side to achieve uniform soak temperature, or the rib region may remain below the target temperature while the thinner shell sections overheat.
3. Preheating before welding is equally critical. For 36 mm P91 material, preheat temperatures of 200–250°C are typically required to prevent hydrogen-induced cracking. The field conditions described in this paper suggest that comprehensive thermal management extends from preheat through welding to post-weld heat treatment.
4. Verification by hardness testing. Post-PWHT hardness of P91 should be ≤ 250 HB (or ≤ 260 HB per some specifications). Hardness measurements at the HAZ, weld metal, and base metal confirm adequate tempering. If hardness exceeds specification, the soak temperature or time was insufficient.

Common Defects and Countermeasures

Study Insights and Reflections

This 1995 paper represents an important milestone in field heat treatment technology for Chinese power plant construction. At that time, many projects relied on conventional gas-fired or electric band heaters that struggled with uniformity on complex geometries. The hybrid infrared-crawler approach demonstrated that multi-method heating strategies could achieve the temperature uniformity required for critical alloy steel components.

The principles described remain relevant today. Modern P91/P92 piping systems in ultra-supercritical units still require PWHT for wall thicknesses above 19 mm, and field conditions often preclude furnace treatment. The fundamental challenge—achieving uniform temperature distribution across thick, geometrically complex sections with limited access—has not changed, even as heating technology has advanced.

One reflection: the paper does not extensively discuss the mechanical properties verification after heat treatment, which is now standard practice per ASME B31.1 and GB/T 20801. Modern practice requires hardness survey, and in some cases, full tensile and Charpy V-notch testing of coupon specimens heat-treated under identical conditions. This gap between 1990s practice and current standards highlights the evolution of quality assurance requirements in power plant construction.

The approach also raises questions about the long-term performance of field-treated tees compared to factory-treated ones. Factory PWHT in controlled furnaces provides superior temperature uniformity and process repeatability. Field treatment, even with advanced heating methods, always carries greater variability. Engineers should consider whether the criticality of the application justifies the inherent limitations of field treatment, or whether design modifications (such as reducing wall thickness or using alternative connection methods) might be preferable.

In summary, this paper provides valuable documentation of a practical solution to a real engineering problem, and its core principles—multi-method heating for uniformity, careful thermocouple placement, and controlled heating/cooling rates—remain foundational to field heat treatment practice for thick-walled alloy steel piping components.