Grades 92 and 23: Weldability Assessment and Long-Term Performances for Power Generation Applications

Abstract

Increased efficiency and emission reduction in modern power plants lead to the use of new advanced materials with enhanced creep strength, with the objective to increase the steam parameters of power plants. With over ten years on market and wide experience related to its use, ASTM Grade 92 is becoming one of the most required materials when high service temperatures are reached (max. 610°C). Its composition, with 9%Cr and 1.5%W, gives rise to martensitic microstructures which offer very high creep strength and long term stability. The improved weldability and creep-strength between 500 and 580°C of the low alloy ASTM Grade 23, as well as a cost advantage over higher Cr materials in this temperature range, make it one of the possible candidates to meet the stringent requirements of modern power plants. Air Liquide Welding (ALW) has optimized and distributes a complete product family for the welding of Grades 23 and 92. TenarisDalmine (TD) focused on the development of Grade 23 tubes and pipes and is working on the development of Grade 92. A deep characterization work of the microstructural evolution and long term creep performances of these high temperature resistant materials was thus undertaken by ALW and TD, in collaboration with the Centro Sviluppo Materiali (CSM). The joint characterization program consisted in the assessment of welded joints creep properties. Welded joints were produced using the gas tungsten (GTAW), shielded metal (SMAW) and submerged arc welding (SAW) processes. Mechanical and creep properties of weldments were measured both in the as welded and post weld heat treated conditions and proper WPS’s were designed in a manner such that industrial production needs were satisfied. Short term creep resistance of cross weld specimens was measured to be within the base material acceptance criteria. Long term base material and cross weld creep performance evaluation are now in progress.

Microstructural evolution and mechanical properties of Grades 23 and 24 creep strength enhanced ferritic steels

“…Unfortunately, there is not extensive metallographic data regarding the failure location for all tests, nor the specimen geometry. In specific instances where failure location was reported, 6,[44][45][46]48,49 the location of failure varied with applied stress. At high stress values, failure was reported in the weld metal or base material.…”

“…Data from Ref. 6,10,[42][43][44][45][46][47] 27 Location of creep specimens taken from cross-weld P23 weldments in Allen et al 49 same heat that was used to make the It is clear that the welds do not provide the same level of creep strength as the base metal. The difference between the base metal and weld rupture times decreases as the PWHT temperature and time is reduced, and the 730-740°C (1346-1364°F )/1 h PWHT gave the best results.…”

“…A number of researchers have investigated the creep behaviour of welds in Grade 23 and have reported cracking in the HAZ at rupture times below the base metal. 6,[42][43][44][45][46][47][48][49] The data contained in Fig. 26 is a compiled set of crossweld performance for Grade 23 weldments from a variety of welding processes including SMAW, GTAW, SAW, FCAW and MAG (i.e.…”

“…GMAW) from. 6,[42][43][44][45][46][47] Material conditions include tests in the as-welded and PWHT cycles at 715-760°C (1319-1400°F) for times ranging from 1 to 3 h. In general, the performance of cross-weld T/P23 tests does not seem to be sensitive welding process or PWHT. The longest creep test in the database is only to ∼10 000 h in life (575°C test temperature).…”

“…A summary of the analysed cross-weld creep data for Grade 23 including the basic details on the tested weldment is provided in Table 10. 6,[42][43][44][45][46] Table 11 provides the regression coefficients for the analysis along the limitations of the analysis. Based on these optimised fit, the 100 000 h rupture lives for 475-575°C (887-1067°F) are compared for the three datasets in Table 12 along with a comparison between the relative creep strengths of the two base metals and the weld strength reduction factors for Grade 23.…”