LEAD ENGINEER – ENGG. (G) – METALLURGY
TATA POWER , MUMBAI
Dis-similar metal weld (DMW) joint or transitions between a Creep strength enhanced ferritic steel (CSEF) and an Austenitic stainless steel (SS) are usually carried out using either an Austenitic Stainless-Steel filler or Nickel base fillers. Austenitic Stainless-Steel fillers are preferred for low temperatures typically less than 400 deg C. For higher temperatures, Nickel based fillers are utilized.
The paper deals with issues in using ERNiCrMo-3 (Inconel 625) filler in combinations of CSEF and Austenitic SS. Multiple Main steam flow nozzles, with steam parameters of 110 kg/cm2 pressure and 540-545 deg C temperature, at a captive power plant had experienced early failures in service. The pipe material was A335 Gr. P91 and Flow nozzle was A182 Gr. 316. The weld procedure employed buttering of P-91 sections using Inconel 625 and PWHT of P-91. The final weld with flow nozzle was carried out using Inconel 625 without PWHT. The failures were in the form of circumferential crack at the interface of P-91 pipe and Inconel 625 butter layer.
The results revealed a significant hardness increase in Inconel 625 weld and observation of creep voids at the interface of P-91 pipe and Inconel 625 butter layer. The phenomenon of precipitation hardening due to formation of secondary precipitates in service at 540-545 deg C led to increase in weld hardness and subsequent creep strain relaxed by formation of creep voids at the interface.
Keywords: Dis-similar metal weld (DMW), Inconel 625, CSEF steel, Creep, precipitation hardening
Multiple failures in main steam flow nozzle were experienced at a captive power plant within 1-1.5 years of service. The process side was steam with parameters of 110 kg/cm2 pressure and 540-545 deg C temperature. The pipe material was A335 Gr. P91 and Flow nozzle was A182 Gr. 316. The weld procedure employed for construction consists of buttering overlay of P91 pipe edge with ERNiCrMo-3 (Inconel 625) followed by PWHT for 2 hours for P91. The final weld with flow nozzle was carried out using Inconel 625 without any PWHT. The failures were in form of circumferential crack just at the interface of P91 and Inconel 625 butter layer covering more than half of pipe circumference as shown in Fig 1. The General arrangement drawing of the Flow nozzle assembly is shown in Fig 2 highlighting the failure location. The outer diameter (ØO) of the pipe was 355.6mm and thickness (W) was 23.83mm.
Figure 1: Failed nozzle
Figure 2: GAD of Flow nozzle
To analyze the failures, investigative methods consisting of visual, chemical analysis, tensile test, optical metallography, SEM, EDS & micro hardness were carried out. The findings of the various test results are highlighted below.
Figure 4: Longitudinal section through nozzle
2.1 VISUAL EXAMINATION
Fig 3 depicts the closeup of the failure location showing the crack at exactly the interface of P91 pipe and Inconel 625 buttering overlay. A longitudinal through wall section cut from the failed sample depicts the crack at the interface and also shows a section of the nozzle element. (Fig 4).
The fractured surface did not reveal any deformation. Individual weld ripples were clearly seen on the fractured surface indicating the crack followed the interface between Inconel buttering overlay weld and pipe parent metal as shown in Fig 5.
Figure 5: Fracture surface towards weld side
Figure 3: Closeup of crack
2.2 CHEMICAL ANALYSIS
Chemical analysis of the pipe parent metal, flow nozzle element and weld indicated that it met the codal requirements of A335 Gr. P91 , A 182 Gr. 316  and SFA 5.14 ERNiCrMo-3  respectively. Table 1 shows the results of chemical analysis.