Good Stress Corrosion Cracking Research Articles

Development of a New High-Strength Steel for Low Pressure Steam Turbine End-Stage Blades

Influenced by the growing share of Renewable Energies, higher flexibility and increased efficiency of fossil power plants as well as improved cost efficiency in production of turbine components are evident market trends. Daily cycling in turbine operations leads to advanced requirements for robust design especially of rotating parts. Low pressure (LP) steam turbine end-stage blades with larger exhaust areas are one lever to increase the efficiency of the turbine by reduction of exhaust losses and also to realize cost-efficient single flow exhaust applications. Consequently, blade steels with improved mechanical properties are required. The results of the development of a new high-strength precipitation-hardening (PH) steel for LP end-stage blade application with significantly enhanced material properties are reported. The paper covers the testing strategy applied and information on crucial material parameters like improved low cycle and high cycle fatigue (HCF) behavior while keeping good stress corrosion cracking (SCC) resistance and corrosion fatigue (CF) properties. Furthermore, first manufacturing experiences and validation results from a full-scale component test rig are presented.

Effect of Cold Rolling on the Transformation Kinetics of Sigma- and Chi- Phases in UNS S32205 Duplex Stainless Steel

Abstract Duplex stainless steels have very good properties including good ductility, moderate toughness and good stress corrosion cracking resistance. These properties can be severely affected by the precipitation of sigma (σ) and chi (χ) phases, in particular ductility and toughness. The effect of cold deformation on the isothermal transformation kinetics of UNS S32205 steel was investigated. The steel was cold rolled up to 64 % reduction in thickness and annealed at 850 °C from 10 min up to 4320 min (3 days). Microstructural characterization used light optical and scanning electron microscopy, energy dispersive spectroscopy, and magnetic measurements. Formation of intermetallics is much faster in the cold-rolled samples. After 4320 min, samples show the same amounts of each phase for all rolling strains. Results have been validated by thermodynamic calculations.

Ａｌ‐Ｚｎ‐Ｍｇ‐Ｃｕ‐Ｌａ系押出合金の引張強度特性と耐応力腐食割れ性に及ぼすＣｒ，ＭｎおよびＭｇ添加の影響

Two groups of the extruded high strength Al-Zn-Mg-Cu-La system alloys, including Al-10 wt.%Zn-1 wt.%Mg-1.5 wt.%Cu-3 wt.%La with various Cr and Mn contents and Al-8 wt.%Zn-1.2 wt.%Cu-4.0 wt.%La with various Mg contents, were developed as the mechanical materials with a good stress corrosion cracking (SCC) resistance. In these alloys, the effects of the additional elements were investigated on the mechanical properties and SCC life. The main results are as follows;(1) The second phase particles which contained Zn and La were observed in the matrix, and dispersed homogeneously. The size of the particles was 5 to 10μm. For the alloys with Cr, dispersoids including Cr were also observed in the matrix. Their size was over 10μm.(2) The maximum value of 568MPa in tensile strength was observed in Al-8 wt.%Zn-1.2 wt.%Cu-4.0 wt.%La with 2.5 wt.%Mg, which was nearly equal to the commercial aluminum alloys7075-T73 and 7475-T6.(3) The excellent resistance of SCC was obtained in these alloys. Their SCC lives were over 15 times as long as that of Al-8 wt.%Zn-1 wt.%Mg master alloy. In conclusion, we developed extruded Al-high Zn-low Mg system alloys with high strength and good SCC resistance.

応力腐食割れ特性に優れた高力Ａｌ‐Ｚｎ‐Ｍｇ‐Ｌａ系合金の開発とその評価 （第４報） 耐応力腐食割れ性に優れた高力Ａｌ‐Ｚｎ‐Ｍｇ‐Ｃｕ‐Ｌａ系押出合金の破壊じん性と第二相粒子の関係

Extruded Al-8wt.% Zn-1-2.5wt.% Mg-1.2wt.% Cu-4wt.% La alloys with high tensile strength and good stress corrosion cracking resistance were developed and their fracture toughness was investigated. The effect of metallurgical parameter on the fracture toughness value was examined. The main results are as follows;(1) The fracture toughness values of the extruded Al-8wt.% Zn-1-2.5wt.% Mg-1.2wt.% Cu-4wt.% La, which were about 20MPa√m without anisotropy, were not influenced by Mg content.(2) The second phase particles in the extruded alloys, which consisted of Zn and La, were estimated quantitatively. The mean diameter was about 4.7μm and the volume fraction of the particles was 1.6-2.3vol.%.(3) The second phase particles consisting of Zn and La in the extruded alloys cracked easily, and became the nucleus of voids when the matrix fractured. The fracture toughness of the extruded alloys was affected by the particles. The fracture toughness value, KIc, was related to the volume fraction of the second phase particles, fv, and the mean diameter of the particles, D, as follows:KIc≈[ζσ0.2E(π/6)1/3D]1/2·fv-1/6∝(DEσ0.2)1/2·fv-1/6where ζ is constant, σ0.2 is 0.2% proof stress and E is Young's modulus.