Strength and toughness of Fe-10ni alloys containing C, Cr, Mo, and Co

173

Hydrogen (H) trap states and binding energies were determined for AERMET 100 (Fe-13.4Co-11Ni-3Cr-1.2Mo-0.2C), an ultrahigh-strength steel using thermal desorption methods. Three major H desorption peaks were identified in the precipitation-hardened microstructure, associated with three distinct metallurgical trap states, and apparent activation energies for desorption were determined for each. The lattice diffusivity (D L ) associated with interstitial H was measured experimentally and verified through trapping theory to yield H-trap binding energies (E b ). Solid-solution elements in AER-MET 100 reduce D L by decreasing the pre-exponential diffusion coefficient, while the activation energy for migration is similar to that of pure iron. M 2 C precipitates are the major reversible trap states, with E b of 11.4 to 11.6 kJ/mol and confirmed by heat treatment that eliminated these precipitates and the associated H-desorption peak. A strong trap state with E b of 61.3 to 62.2 kJ/mol is likely associated with martensite interfaces, austenite grain boundaries, and mixed dislocation cores. Undissolved metal carbides and highly misoriented grain boundaries trap H with a binding energy of 89.1 to 89.9 kJ/mol. Severe transgranular hydrogen embrittlement in peak-aged AERMET 100 at a low threshold-stress intensity is due to H repartitioning from a high density of homogeneously distributed and reversible M 2 C traps to the crack tip under the influence of high hydrostatic tensile stress.

Section: Trap State 1: Peak 1a In As-quenched Alloymentioning

confidence: 97%

Hydrogen trap states in ultrahigh-strength AERMET 100 steel

Gangloff

Scully

2004

173

“…[3,4,34,35] The effect of Cr on the acceleration of aging may be explained by the relatively rapid growth of As-quenched microstructures after austenitizing at 1000 2. Fracture behavior Variations in impact toughness with aging temperature for the various austenitizing conditions are shown in Figure 13.…”

Section: B Effect Of Austenitizing Treatments (Mo-w-cr-co-ni Steel)mentioning

confidence: 99%

“…These More recently, the effect of alloying additions on the steels are based on the design of HY180 (8Co-10Ni-2Cr-secondary hardening behavior has been systematically ana1Mo-0.1C). [3,4] lyzed for a number of alloy systems by means of the stepwise The precipitates providing the secondary hardening are alloying additions of Cr, Co, and Ni. [8,9,10] The systems anafine M 2 C-type carbides that are formed by the dissolution lyzed range from the basic system of the Fe-C-Mo and Feof M 3 C-type cementite during aging at temperatures near C-W ternary alloys, which have strong and weak effects on 500 ЊC for the high Co-Ni secondary hardening alloy steels.…”

Section: Introductionmentioning

confidence: 99%

Effects of alloying additions and austenitizing treatments on secondary hardening and fracture behavior for martensitic steels containing both Mo and W

Lee

Kwon

Yang³

2001

The effects of alloying additions and austenitizing treatments on secondary hardening and fracture behavior of martensitic steels containing both Mo and W were investigated. The secondary hardening response and properties of these steels are dependent on the composition and distribution of the carbides formed during aging (tempering) of the martensite, as modified by alloying additions and austenitizing treatments. The precipitates responsible for secondary hardening are M 2 C carbides formed during the dissolution of the cementite (M 3 C). The Mo-W steel showed moderately strong secondary hardening and delayed overaging due to the combined effects of Mo and W. The addition of Cr removed secondary hardening by the stabilization of cementite, which inhibited the formation of M 2 C carbides. The elements Co and Ni, particularly in combination, strongly increased secondary hardening. Additions of Ni promoted the dissolution of cementite and provided carbon for the formation of M 2 C carbide, while Co increased the nucleation rate of M 2 C carbide. Fracture behavior is interpreted in terms of the presence of impurities and coarse cementite at the grain boundaries and the variation in matrix strength associated with the formation of M 2 C carbides. For the Mo-W-Cr-Co-Ni steel, the double-austenitizing at the relatively low temperatures of 899 to 816 ЊC accelerated the aging kinetics because the ratio of Cr/(Mo ϩ W) increased in the matrix due to the presence of undissolved carbides containing considerably larger concentrations of (Mo ϩ W). The undissolved carbides reduced the impact toughness for aging temperatures up to 510 ЊC, prior to the large decrease in hardness that occurred on aging at higher temperatures.

“…When aging Aermet 100 at higher temperatures, these exceptional mechanical properties are quickly lost due to overaging, giving this alloy a very narrow heat-treatment window for aging. [22][23][24] To reduce the effect of overaging during postweld heat treatment (PWHT), Aermet 100 can be joined in a partially aged condition.…”

Section: Methodsmentioning

confidence: 99%

Residual Stresses in Inertia-Friction-Welded Dissimilar High-Strength Steels

Moat

Hughes

Steuwer

et al. 2009

The welding of dissimilar alloys is seen increasingly as a way forward to improve efficiencies in modern aeroengines, because it allows one to tailor varying material property demands across a component. Dissimilar inertia friction welding (IFW) of two high-strength steels, Aermet 100 and S/CMV, has been identified as a possible joint for rotating gas turbine components and the resulting welds are investigated in this article. In order to understand the impact of the welding process and predict the life expectancy of such structures, a detailed understanding of the residual stress fields present in the welded component is needed. By combining energy-dispersive synchrotron X-ray diffraction (EDSXRD) and neutron diffraction, it has been possible to map the variations in lattice spacing of the ferritic phase on both sides of two tubular Aermet 100-S/ CMV inertia friction welds (as-welded and postweld heat-treated condition) with a wall thickness of 37 mm. Laboratory-based XRD measurements were required to take into account the variation in the strain-free d-spacing across the weld region. It was found that, in the heataffected zone (HAZ) slightly away from the weld line, residual stress fields showed tensile stresses increasing most dramatically in the hoop direction toward the weld line. Closer to the weld line, in the plastically affected zone, a sharp drop in the residual stresses was observed on both sides, although more dramatically in the S/CMV. In addition to residual stress mapping, synchrotron XRD measurements were carried out to map microstructural changes in thin slices cut from the welds. By studying the diffraction peak asymmetry of the 200-a diffraction peak, it was possible to demonstrate that a martensitic phase transformation in the S/CMV is responsible for the significant stress reduction close to the weld line. The postweld heat treatment (PWHT) chosen to avoid any overaging of the Aermet 100 and to temper the S/CMV martensite resulted in little stress relief on the S/CMV side of the weld.