ISSN:0960-3409    

DOI:10.1080/09603409.1994.11689464

出版商:Taylor&Francis    

年代:1994

数据来源: Taylor

摘要:

The creep rapture behaviour of 2.25Cr—1Mo steel in air and in a salt mixture was studied. The salt coating, which can form a liquid phase at the test temperatures, increased the creep rate and reduced the rupture life of the material. The coating reduced the available cross-section of the material by removing the surface layers, thereby resulting in a reduction of the rupture life. Cross-sections of coated samples showed an outer oxide layer comprising oxide of the metal and precipitates of sulphide at the metal/oxide interface. This subsurface penetration of the corrodants was responsible for the early failure of the coated samples. This is typical of hot corrosion mechanisms. The formation of various carbides like M23C6and M6C, as observed by transmission electron microscopy, during creep reduced the creep strength of the material both in air and in the coated state. Increasing temperature enhanced the formation of these carbides with a consequent decrease in creep strength. Applied stress did not seem to play much of a role in the degree of carbide precipitation.

ISSN:0960-3409    

DOI:10.1080/09603409.1994.11689465

出版商:Taylor&Francis    

年代:1994

数据来源: Taylor

摘要:

This paper describes a detailed investigation which was specifically aimed at providing a simple practical assessment procedure which identified reverse temper embrittlement (RTE) in a series of high-pressure (HP) and intermediate-pressure (IP) CrMoV steel turbine bolts from two separate 120 MW sister units which had been subjected to 120 000 h of service. A small section of material was removed from a non-critical location from all 182 bolts. From this section the chemical composition, average hardness and prior austenite grain size were assessed. The toughness of selected bolts was measured by either (i) Charpy impact testing or (ii) Auger electron spectroscopy. It was established that grain size and bulk phosphorus content were the primary factors which consistently correlated with the embrittled condition. An embrittlement estimative diagram was established by plotting grain sizedversus percentage phosphorus. This portrayed two distinct regions, namely an embrittled region and a non-embrittled region, which were separated by a critical interface which could be described by the expressiond(%P) =CwhereCis a constant. The value ofCis, however, strongly dependent upon the level of accumulated bolt strain during service. Indeed, the higher the value of average bolt strainεAV, the smaller the numerical value ofC, i.e. with increasingεAVembrittlement was promoted in bolts with finer grain sizes and lower phosphorus levels. Indeed, it was projected that atεAV= 0.5% nearly all the bolts in the present investigation would have undergone embrittlement. From a creep damage assessment conducted on a selection of bolts in the thread root location it was observed that, at average strain levels approaching 0.5%, bolts with grain sizes of about 20μm will have suffered RTE and a measurable amount of secondary creep damage. The limited amount of data suggested that, for coarser grain sizes, creep damage or grain boundary cavitation occurred at lower accumulated strains. Finally it was shown that, in the few instances where secondary creep damage was recorded, the accumulated strain at the thread root location was 2.7 times higher than the average bolt strainεAVobtained by bolt length measurements.

ISSN:0960-3409    

DOI:10.1080/09603409.1994.11689466

出版商:Taylor&Francis    

年代:1994

数据来源: Taylor

摘要:

Postservice examination of a thick-section AISI 347/AISI P22 transition joint revealed wedge-type creep damage at prior austenite grain boundaries in the low alloy steel heat-affected zone close to the fusion boundary. This damage had linked to form axial grain boundary microcracks oriented at 45°to the weld profile. A circumferential macrocrack had formed in these regions. The cracking observed is explained in terms of the high stresses generated due to the mismatch in coefficients of thermal expansion between the high and low alloy steels. Examination of the detail weldment microstructures was undertaken using optical and electron microscopy. Some success in predicting operating temperatures has been achieved from analysis of the carbides present.

ISSN:0960-3409    

DOI:10.1080/09603409.1994.11689467

出版商:Taylor&Francis    

年代:1994

数据来源: Taylor

摘要:

This paper summarizes the work undertaken in a five year collaborative project aimed at developing improved materials and coatings for exhaust valves in diesel engines for marine service, operating on residual fuel oils. Eighty candidate materials were assessed using a range of laboratory tests designed to simulate various aspects of engine conditions. Of particular interest to this paper are those relating to high temperature corrosion and corrosive wear in this diesel engine environment. Following these laboratory tests, prototype engine valves were produced in 15 different combinations of the most promising materials and their performance was compared in severe engine trials with that of the best currently used commercial materials. Some experimental nickel alloys, coatings and hardfacings showed considerable promise together with a cheaper, coated iron-based valve alloy. In addition to, and as a result of, these material evaluation programmes a life prediction model has been devised which gives engine valve lives as a function of alloy composition and valve seat operating temperature for engines operating on residual fuels oils.

ISSN:0960-3409    

DOI:10.1080/09603409.1994.11689468

出版商:Taylor&Francis    

年代:1994

数据来源: Taylor

摘要:

With regard to the cracking (disbonding) which occurs at the fusion boundary of austenitic overlays and the base metal of high-temperature, high-pressure hydrogen reactors, the accumulation of hydrogen at the fusion boundary subsequent to cooling from a high to a normal temperature, which is considered to be the largest causative factor of such cracking, has been evaluated by numerically solving diffusion equations using differential calculus. It has been shown that the amount of accumulated hydrogen can be expressed by applying a simple, approximate formula which incorporates the service pressure of the hydrogen, service temperature, thickness of overlay and base metal, cooling rate, hydrogen solubility, and hydrogen diffusion coefficients.

ISSN:0960-3409    

DOI:10.1080/09603409.1994.11689469

出版商:Taylor&Francis    

年代:1994

数据来源: Taylor

摘要:

The interaction between surface engineering and corrosion resistance of high temperature materials is reviewed. A brief description is given of high temperature corrosion, while criteria for surface engineering are outlined. The most important classes of surface treatments utilized technologically to provide high temperature corrosion protection, namely surface alloying, diffusion coatings and also metallic and ceramic overlay coatings, are discussed. The various ways in which surface engineering can interact with high temperature corrosion resistance are considered. Thus, the main purpose of the surface layer can be solely to afford protection, but in other cases, such as with thermal barrier coatings, the surface layers have another primary task, but additionally provide corrosion resistance. Finally, the important role of surface modification techniques, in particular ion implantation, as a research tool for corrosion science is highlighted.

ISSN:0960-3409    

DOI:10.1080/09603409.1994.11689470

出版商:Taylor&Francis    

年代:1994

数据来源: Taylor

摘要:

An earlier modification of the Paris law for the growth of deep cracks in the linear elastic fracture mechanics regime is extended to include a term enabling the prediction of cyclic crack growth rates at low frequencies. The relation requires (i) a reference growth law under continuous cycling at the appropriate elevated temperature and (ii) a specified, dimensionless degradation term, defined asDc, the creep/oxidation damage per cycle, which increases as the applied frequency decreases or as the dwell time at peak load is prolonged. The relationship is validated against data from the previous analysis on low alloy ferritic and austenitic steels in the range 538–650°C and against further published results on Ni-based alloys at temperatures up to 700°C. It appears that for the former series oxidation is the dominant damaging mode, whereas a linear creep damaging mechanism is manifest in the Ni-based alloys. Moreover, levels of cyclic damage in terms ofDcare higher in the latter, ranging between 10−3and 5×10−1compared with 10−4to 5×10−2for the steels. A brief comparison is made with damage factors arising from the high strain fatigue deformation mode at elevated temperatures and other models for the prediction of frequency effects are discussed.

ISSN:0960-3409    

DOI:10.1080/09603409.1994.11689471

出版商:Taylor&Francis    

年代:1994

数据来源: Taylor