Segregation-Induced Enhancement of Low-Temperature Tensile Ductility in a Cast High-Nitrogen Austenitic Stainless Steel Exhibiting Deformation-Induced α′ Martensite Formation

Abstract

In spite of the formation of a high fraction of deformation-induced a¢ martensite, the tensile elongation of a cast high-nitrogen austenitic stainless steel was found to enhance at lower temperatures, a behavior deviating from that exhibited by wrought and homogenized austenitic stainless steels. The observed behavior was explained by the presence of microstructural regions with different stabilities with respect to deformationinduced a¢ martensite formation caused by the segregation of alloying elements. Tensile elongation near room temperature of low stacking fault energy (SFE) austenitic steels including stainless, [1][2][3][4][5][6][7][8][9] high Mn, [10][11][12][13] and high Ni [14] steels varies in three temperature regimes as follows. At the highest temperature range (regime I), tensile elongation remains more or less constant or exhibits a weak temperature dependence. At intermediate temperatures (regime II), elongation increases as the temperature decreases. The enhancement of ductility at reduced temperatures in regime II is commonly attributed to such deformationinduced microstructural changes as e martensite formation, the e-TRIP effect, [15] and deformation twinning, the TWIP effect.[2] These are common microstructural features of deformed austenitic stainless steels [16,17] and high Mn [12,18,19] steels and are thought to be consequences of high glide planarity. Enhanced glide planarity caused by reduced cross slip of screw dislocations at lower temperatures has been correlated with an underlying temperature dependence of SFE.[20] The latter dependence is available for many fcc metals and alloys including austenitic steels. [20][21][22][23][24] The enhancement of tensile elongation at lower temperatures in regime II of austenitic stainless steels is interrupted at the temperature below which the deformation-induced formation of a¢ martensite is enabled, namely below the M d cfia¢ temperature. The loss of ductility caused by the deformation-induced formation of a¢ martensite initiates the regime III of elongation, characterized by reduced elongations at lower temperatures. The large number of investigations in support of the detrimental effect of a¢ martensite formation on the tensile elongation of metastable austenitic stainless steels [1][2][3][4][5][6][7][10][11][12] indicates that any possible contribution to the ductility of a TRIP effect would not be large enough to prevent the loss of ductility below the M d cfia¢ temperature. This has been also suggested by the modeling of the a¢ TRIP effect contribution to the ductility.[25] The present study demonstrates how compositional inhomogeneities in a cast high-nitrogen austenitic stainless steel can cause deviation from the behavior described above.

Introductory Chapter: Why Austenitic Stainless Steels are Continuously Interesting for Science?

“…The influence of alloying elements on the SFE of austenitic stainless steels can be deduced from their influence on the M d γ → α′ temperature. Therefore, relationships giving the compositional dependence of Md γ → α′ temperature can be used as guidelines for the design of austenitic stainless steels with improved properties [4,5].…”

Microstructural Evolution of an Al-Alloyed Duplex Stainless Steel During Tensile Deformation Between 77 K and 473 K (−196 °C and 200 °C)

“…At temperatures around the DBTT, however, a sharp decrease in the toughness and ductility occurs as the temperature decreases. [40] The elongation of metastable austenitic steels, on the other hand, is characterized by a maximum in the vicinity of the M d cfia¢ temperature, [41][42][43] namely the highest temperature at which the austenite to martensite transformation can be induced by plastic deformation. [44] The loss of ductility below the M d cfia¢ temperature is due to the deformation-induced formation of a¢-martensite.…”

Thermal and deformation-induced phase transformation behavior of Fe–15Cr–3Mn–3Ni–0.1N–(0.05–0.25)C austenitic and austenitic–martensitic cast stainless steels

“…10. In this respect, the formation of martensite preferentially occurs in regions containing a lower concentration of alloying elements which are usually near the dendritic cores [48]. As a result of a more pronounced surface decarburization and/or denitriding in the NC25 steel which was heat treated at a higher temperature than the NC20 alloy (1200°C vs 1150°C), the martensitic layer at the surface of the NC25 alloy was thicker and had a lower hardness compared to the NC20 alloy.…”

“…The temperature dependence of the SFE which controls the dissociation distance of partial dislocations and therefore the ease of the cross slip then decides the extent of glide planarity [63][64][65]. The latter parameter in turn influences the strain-hardening rate and therefore the uniform tensile elongation of austenitic steels [48].…”