Maribel L. Saucedo-Muñoz

Creep property measurement of service-exposed SUS 316 austenitic stainless steel by the small-punch creep-testing technique

The creep properties for SUS 316 HTB austenitic stainless steel were evaluated by using the small-punch creep test at 650 °C for loads of 234, 286, 338, 408, and 478 N and at 700 °C for loads of 199 and 234 N. The creep curves, determined by means of the small-punch creep test, were similar to those obtained from a conventional uniaxial creep test. That is, they exhibited clearly the three creep stages. The width of secondary creep stage and rupture time t r decreased with the increase in testing load level. The creep rupture strength for the service-exposed material was lower than that of the as-received material at high testing loads. However, the creep resistance behavior was opposite at relatively low load levels. This difference in creep resistance was explained on the basis of the difference in the creep deformation and microstructural evolution during tests. It was also found that the ratio between the load of small-punch creep test and the stress of uniaxial creep test was about 1 for having the same value of creep rupture life.

Effect of microstructure evolution on fracture toughness in isothermally aged austenitic stainless steels for cryogenic applications

Abstract Two types of austenitic stainless steels JJ1 and JN1 were isothermally aged at temperatures from 600°C to 900°C for 10–1000 min in order to study the microstructural evolution and its effect on fracture toughness at cryogenic temperatures. These steels were developed for applications in the superconducting magnets of a fusion experimental reactor by the Japan Atomic Energy Research Institute. The Charpy V-Notch (CVN) fracture energy at 77 K showed a significant decrease with aging time for both steels. The intergranular precipitation of carbides and nitrides is responsible for the fracture toughness deterioration. The scanning electron microscope (SEM) fractographs showed an intergranular brittle fracture and its fraction also increased with aging time and temperature. The presence of a more abundant intergranular precipitation resulted in a more rapid decrease in fracture toughness with aging time in JN1 steel due to its higher content of C and N, compared to that of JJ1 steel. The volume fraction of precipitates can be uniquely correlated with the reduction in toughness.

Correlationship between JIC and equivalent fracture strain determined by small-punch tests in JN1, JJ1 and JK2 austenitic stainless steels

Abstract Small-punch (SP) and fracture toughness tests were conducted at 4 and 77 K on newly developed cryogenic austenitic stainless steels after isothermal aging. Equivalent fracture strain eqf was determined by measuring the reduction in thickness of SP specimens. A linear dependence of JIC on eqf was found for these new materials. Regression analysis of experimental data produced the linear relation: J IC =1304.1e qf +8.09 ( kJ m −2 ) . An analysis of the correlation was pursued on the basis of material properties and evaluation method of JIC. The results showed that the slope value was influenced by the JIC evaluation method. This linear relation enables the prediction of JIC to be made from the equivalent fracture strain eqf by conducting small punch tests. Thus, it is expected that the SP testing method provides a useful basis for assessing the degradation of fracture toughness in small areas, such as heat-affected zones in welded components.

Microstructure characterization of phase transformations in a Zn–22 wt%Al–2 wt%Cu alloy by XRD, SEM, TEM and FIM

Abstract The microstructure evolution in a Zn–22 wt%Al–2 wt%Cu alloy during aging was followed, using X-ray diffraction, scanning electron microscopy, transmission electron microscopy and field ion microscopy. Samples were solution treated at 623 K for 1 week and then some of them annealed and the others quenched. Both annealed and quenched samples were aged at 373, 423 and 473 K for times from 3.6 (0.04 days) to 1800 ks (20.8 days). The X-ray patterns showed that the η phase decomposed according to the following reaction η→η+α during aging. This was also confirmed by TEM and SEM results, which showed first the spinodal decomposition of the η phase and then the formation of a coarse lamellar structure, composed of the α and η phases. The X-ray analysis of aged samples also showed the occurrence of a four-phase reaction α+ϵ→τ+η. The τ phase was widely recognized in SEM, TEM and FIM results. Additionally, the XRD and TEM results indicated that the formation of the equilibrium ordered τ′ phase is preceded by that of the disordered τ phase.

Development of a Multiple Linear Regression Model to Estimate the Ductile-Brittle Transition Temperature of Ferritic Low-Alloy Steels Based on the Relationship Between Small Punch and Charpy V-Notch Tests

The transition temperatures of Cr-0.5Mo, Cr-Mo, and Cr-Mo-V steels were determined using the Charpy V-notch (CVN) and the small punch (SP) tests. It was confirmed that there was a linear correlation between the transition temperature of ductile-brittle behavior determined by the Charpy V-notch test and that obtained from the small punch test. However, the estimation of CVN transition temperature by means of this linear equation is not completely reliable because of the large experimental scatter of data. In order to improve the reliability of the transition temperature estimation, a multiple linear regression (MLR) analysis was conducted to evaluate the effect of different variables of the manufacturing process and service conditions. This analysis permitted the determination of the following regression equation: CVNDBTT = 1.35 SPDBTT − 0.84 × 10 3 d − 1 ∕ 2 + 326 . This equation enables one to assess more accurately the transition temperature corresponding to the Charpy V-notch test using that of the small punch test and the austenitic grain size, expressed by d − 1 ∕ 2 .

Effect of Active and Nonactive Fluxes on the Mechanical Properties and Microstructure in Submerged-Arc Welds of A-36 Steel Plates

A study of the effect of active and nonactive fluxes on the mechanical properties and microstructure of submerged-arc welds for steel plates was carried out for the Submerged-Arc Welding (SAW) of A-36 Steel Plates. The nonactive flux promoted the formation of pearlite and ferrite in the weld having the highest toughness and ductility. In contrast, the active fluxes with Cr and Mo promoted the formation of acicular ferrite and fine carbides in the welds showing the highest tensile strength and hardness.

Evaluation of thermal aging embrittlement of austenitic stainless steels JN1, JJ1, and JK2 by cryogenic small-punch testing

Small-punch tests were conducted at 4, 77, and 293 K on three newly developed cryogenic austenitic stainless steels, JN1, JJ1, and JK2, which were solution treated, water-quenched, and then aged at 923, 973, 1023, and 1073 K for 5 h. Small-punch test energy was employed for the evaluation of the aging-induced embrittlement behavior in these materials. An SEM analysis of the fracture surface for the solution-treated steel specimens indicated a ductile fracture, having the highest SP test energy values. On the contrary, intergranular brittle fracture was observed in aged specimens. The small-punch test energy of materials decreased significantly as the aging process progressed. The highest and lowest decrease in small-punch test energy with aging temperature occurred in JN1 and JK2 steels, respectively. The decrease in small-punch test energy was shown to follow appropriately the aging-induced embrittlement in these materials. The difference in aging-induced embrittlement behavior for these steels was explained on the basis of the volume fraction of intergranular precipitates in aged samples.

Mechanical milling induced preferred orientation in a Zn-Al based alloy

Abstract Mechanical milling resulted in the preferred orientation of the Zn-rich metastable hep eη′FC and η′γ phases in the furnace cooled (FC) eutectoid Zn-Al based alloy filings, after hot rolling or elevated temperature tensile-creep. The milling induced preferred orientation for the Zn-rich e phase, and theeη′FC and η′γ phases in the eutectoid alloy were at (0002) and 10l0 crystal planes respectively, which reverted the original, 0 h milling state orientation as the directional external stress was destroyed during further milling.

Effect of Precipitation on Cryogenic Toughness in N-Containing Austenitic Stainless Steels

Three types of austenitic stainless steels JK2, JJ1 and JN1 were isothermally aged at temperatures from 600 to 900°C for 10 to 1000 minutes in order to study the microstructural evolution and its effect on the fracture toughness at cryogenic temperatures. The Charpy V-Notch fracture energy at 77 K showed a significant decrease with aging time in JJ1 and JN1 steels because of their higher contents of C and N. In contrast, the fracture energy corresponding to the aged JK2 steel decreased gradually with aging time. The abundant intergranular precipitation of carbides and nitrides seems to be the responsible for the fracture toughness deterioration in the aged JJ1 and JN1 steels. On the other hand, the intergranular precipitation of carbides was less abundant in the aged JK2 steel. The scanning electron microscope fractographs of the CVN test specimens corresponding to the aged JJ1 and JN1 steels showed mainly an intergranular brittle fracture and its fraction increased with aging time and temperature. In general, the presence of a more abundant intergranular precipitation resulted in a more rapid decrease in toughness with aging time.

Precipitation analysis of as-cast HK40 steel after isothermal aging

As-cast HK40 steel was aged at 700, 800, or 900°C for times as long as 2000 h. Microstructural characterization showed that the primary M7C3 carbide network contained a substantial content of manganese, in agreement with the microsegregation of manganese calculated by Thermo-Calc using the Scheil–Gulliver module. The dissolution of primary carbides caused the solute supersaturation of austenite and subsequent precipitation of fine M23C6 carbides in the austenite matrix for aged specimens. During prolonged aging, the carbide size increased with increasing time because of the coarsening process. A time–temperature–precipitation diagram for M23C6 carbides was calculated using the Thermo-Calc PRISMA software; this diagram showed good agreement with the experimental growth kinetics of precipitation. The fine carbide precipitation caused an increase in hardness; however, the coarsening process of carbides promoted a decrease in hardness. Nanoindentation tests of the austenite matrix indicated an increase in ductility with increasing aging time.