Type IV cracking in ferritic power plant steels
TL;DR: In this paper, the authors assess the relationship between the composition and microstructure of 9-12 Cr steels, the welding and fabrication procedures and how these factors translate into a propensity for type IV failures.
Abstract: There have been concerted world wide efforts to develop steels suitable for use in efficient fossil fired power plants. Ferritic alloys containing between 9 and 12 wt-% chromium are seen as the most promising materials in this respect, especially for thick walled components such as headers and the main steam pipe in boilers. However, the performance of the improved steels has often not been realised in service, because premature failures occur in the heat affected zone of welded joints in a phenomenon referred to as type IV cracking. This review assesses the relationship between the composition and microstructure of 9–12 Cr steels, the welding and fabrication procedures and how these factors translate into a propensity for type IV failures.
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TL;DR: The suppression of particle coarsening during creep and the maintenance of a homogeneous distribution of M23C6 carbides near prior austenite grain boundaries are effective for preventing the long-term degradation of creep strength and for improving long- term creep strength by the addition of boron.
420 citations
TL;DR: In this paper, the authors assess the relationship between the composition and microstructure of 9-12 Cr steels, the welding and fabrication procedures and how these factors translate into a propensity for type IV failures.
Abstract: There have been concerted world wide efforts to develop steels suitable for use in efficient fossil fired power plants. Ferritic alloys containing between 9 and 12 wt-% chromium are seen as the most promising materials in this respect, especially for thick walled components such as headers and the main steam pipe in boilers. However, the performance of the improved steels has often not been realised in service, because premature failures occur in the heat affected zone of welded joints in a phenomenon referred to as type IV cracking. This review assesses the relationship between the composition and microstructure of 9–12 Cr steels, the welding and fabrication procedures and how these factors translate into a propensity for type IV failures.
301 citations
TL;DR: In this article, the authors review metallurgical issues that arise in ferritic steel welds, relate these to the difficulties in calculating residual stresses, and propose a method to estimate residual stresses in the case of ferritic alloys.
Abstract: Many of the degradation mechanisms relevant to power plant components can be exacerbated by stresses that reside within the material. Good design or structural integrity assessments require therefore, an accounting of residual stresses, which often are introduced during welding. To do this it is necessary to characterise the stresses, but this may not be possible in thick components using non-destructive methods. These difficulties, and a paucity of relevant engineering data, have led to an increasing emphasis on the development and validation of suitable modelling tools. Advances are prominent in the estimation of welding residual stresses in austenitic stainless steels. The progress has been less convincing in the case of ferritic alloys, largely due to the complexities associated with the solid state phase transformations that occur in multipass welding. We review here the metallurgical issues that arise in ferritic steel welds, relate these to the difficulties in calculating residual stresses,...
188 citations
TL;DR: A critical review of the main results obtained to date in the secondary and tertiary stages of creep is presented in this article, and the advantages and disadvantages of each method are discussed.
Abstract: The assessment of creep damage in steels employed in the power generation industry is usually carried out by means of replica metallography, but the several shortcomings of this method have prompted a search for alternative or complementary non-destructive techniques, ranging from ultrasonic to electromagnetic methods, hardness measurements and nuclear techniques. A critical review of the main results obtained to date in the secondary and tertiary stages of creep is presented in this paper, and the advantages and disadvantages of each method are discussed. Ultrasonic and potential drop techniques appear to be the most promising, but further research is needed before they are fully mature for deployment in the field.
183 citations
TL;DR: In this article, microstructure evolution in P91 steel and their weldments are reviewed in as-virgin and heat treatment and creep exposure condition, and the role of grain coarsening, Cr/Fe ratio, lath widening and dislocation density on creep rupture life of base metal and weldments is discussed.
155 citations
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TL;DR: In this paper, the authors assess the relationship between the composition and microstructure of 9-12 Cr steels, the welding and fabrication procedures and how these factors translate into a propensity for type IV failures.
Abstract: There have been concerted world wide efforts to develop steels suitable for use in efficient fossil fired power plants. Ferritic alloys containing between 9 and 12 wt-% chromium are seen as the most promising materials in this respect, especially for thick walled components such as headers and the main steam pipe in boilers. However, the performance of the improved steels has often not been realised in service, because premature failures occur in the heat affected zone of welded joints in a phenomenon referred to as type IV cracking. This review assesses the relationship between the composition and microstructure of 9–12 Cr steels, the welding and fabrication procedures and how these factors translate into a propensity for type IV failures.
301 citations
TL;DR: In this article, a methodology is described for the calculation of complex precipitation reactions over periods extending many tens of years. And the estimation of the creep rupture stress using a neural network technique is described.
Abstract: Creep resistant steels must be reliable over very long periods of time in severe environments. Their microstructures have to be very stable, both in the wrought and in the welded states. This paper reviews the quantitative methods for the design of steels for elevated temperature applications. A methodology is described for the calculation of complex precipitation reactions over periods extending many tens of years. However, microstructure alone is not enough in the design of alloys. The estimation of the creep rupture stress using a neural network technique is described in the second part of this review. The calculation of the influence of solute-elements on the self-diffusivity of iron, which features in many creep equations, is an emerging area in alloy design. The methodology for such calculations is reviewed in the final section of the paper.
206 citations
TL;DR: The distribution and precipitated amounts of M23C6 carbides and MX-type carbonitrides with decreasing carbon content from 0.16 to 0.002 mass pct in 9Cr-3W steel, which is used as a heat-resistant steel, has been investigated in this article.
Abstract: The distributions and precipitated amounts of M23C6 carbides and MX-type carbonitrides with decreasing carbon content from 0.16 to 0.002 mass pct in 9Cr-3W steel, which is used as a heat-resistant steel, has been investigated. The microstructures of the steels are observed to be martensite. Distributions of precipitates differ greatly among the steels depending on carbon concentration. In the steels containing carbon at levels above 0.05 pct, M23C6 carbides precipitate along boundaries and fine MX carbonitrides precipitate mainly in the matrix after tempering. In 0.002 pct C steel, there are no M23C6 carbide precipitates, and instead, fine MX with sizes of 2 to 20 nm precipitate densely along boundaries. In 0.02 pct C steel, a small amount of M23C6 carbides precipitate, but the sizes are quite large and the main precipitates along boundaries are MX, as with 0.002 pct C steel. A combination of the removal of any carbide whose size is much larger than that of MX-type nitrides, and the fine distributions of MX-type nitrides along boundaries, is significantly effective for the stabilization of a variety of boundaries in the martensitic 9Cr steel.
204 citations
TL;DR: In this paper, the effect of fine precipitation and subsequent coarsening of Fe2W Laves phase on the creep deformation behavior was investigated for simple 9Cr-W steels containing 0, 1, 2, and 4 wt pct W.
Abstract: The effect of fine precipitation and subsequent coarsening of Fe2W Laves phase on the creep deformation behavior was investigated for simple 9Cr-W steels containing 0, 1, 2, and 4 wt pct W. After tempering, the specimens were subjected to creep tests at 823, 873, and 923 K for up to 15,000 hours. The precipitation of Fe2W Laves phase takes place during creep at boundaries from the supersaturated solid solution of the high-W steels, the 9Cr-2W and 9Cr-4W steels, but not in the low-W steels, the 9Cr-0W and 9Cr-1W steels. The fine precipitation of Fe2W Laves phase decreases the creep rate in the primary or transient creep region, while the subsequent large coarsening of Fe2W Laves phase reduces the precipitation strengthening and promotes the acceleration of creep rate in the tertiary or acceleration creep region after reaching a minimum creep rate. The change in shape of creep rate curves with stress and temperature is explained by taking fine precipitation and subsequent coarsening of Fe2W Laves phase into account.
134 citations
TL;DR: In this paper, the authors summarized recent developments in the deep-hole method for measuring residual stresses in thick-section welds and compared the results with measurements obtained from several locations in the complex geometry.
Abstract: Significant levels of residual stress are developed in the production of thick-section steel welds. Residual stress can be measured by a variety of methods, although few offer the ability to determine the spatial distribution completely through the thickness. This paper summarizes recent developments in the deep-hole method for measuring residual stresses in thick-section welds. Experimental results obtained from a variety of welded steel components are described. The measurement method has also been used to provide validation for finite element simulations of residual stresses in a welded cylinder-to-nozzle stainless steel component. The paper describes the finite element weld analysis and compares the results with measurements obtained from several locations in the complex geometry. Overall, there is good agreement between the predicted and measured distributions of residual stress, but the magnitudes of predicted stress tend to be greater.
133 citations