The mechanism of nitrogen addition to AISI H13 die steel is proposed and supported using thermodynamic calculations in addition to observed changes in precipitate, microstructure, crystal structure, and macroproperties. The results indicate that the average impact toughness ak of the novel nitrogen H13 steel is maximally 17.6 J cm−2 and minimally 13.4 J cm−2. These values result in die steel that reaches premium grade and approximate the superior grade as specified in NADCA#207-2003, additionally the hardness is improved 3–5HRC. Experimental findings indicate that the residual V(C,N) particles undissolved during nitrogen H13 steel austenitizing by quenching helps to suppress growth of original austenitic crystal grains, this in turn results in finer martensitic structures after quenching. In the subsequent tempering process all N atoms are dissolved in the solid state matrix a result of C atoms displacing N atoms in V(C,N). Solid dissolution of N atoms produces a distorted lattice of Fe matrix which results in an increase in the hardness of the steel. Additionally this displacement reaction is important for slow growth of secondary particles in nitrogen H13 steel during the tempering process which helps to increase impact toughness compared to its nitrogen-free counterpart given the same condition of heat-treatment.

Mechanism of improvement on strength and toughness of H13 die steel by nitrogen

It is known that mechanical properties of hot working die steel deteriorate during service due to dynamic recovery, recrystallization and plastic deformation of tempered martensite. In order to clarify the associated microstructure evolution and fracture mechanism, a systematic investigation was carried out on a widely used hot working die steel. Samples were tensile deformed at different temperatures ranging from 25 ℃ to 640 ℃ and examined using SEM and TEM. Results obtained have shown that mechanical properties as well as fracture mode change with increasing testing temperature. These phenomena have been elaborated together with microstructure evolution during high temperature deformation. Deformation mechanisms and roles of different carbides in crack formation and propagation have been discussed.

Microstructure evolution and fracture mechanism of H13 steel during high temperature tensile deformation

The friction and wear behaviors of a new hot-work die steel, SDCM-SS, were studied at high temperature under dry air conditions. The wear mechanism and microstructural characteristics of the SDCM-SS steel were also investigated. The results showed that the SDCM-SS steel had greater wear resistance compared with H13 steel; this was owed to its high oxidizability and temper stability. These features facilitate the generation, growth, and maintenance of a tribo-oxide layer at high temperature under relatively stable conditions. The high oxidizability and thermal stability of the SDCM-SS steel originate from its particular alloy design. No chromium is added to the steel; this ensures that the material has high oxidizability, and facilitates the generation of tribo-oxides during the sliding process. Molybdenum, tungsten, and vanadium additions promote the high temper resistance and stability of the steel. Many fine Mo2C and VC carbides precipitate during the tempering of SDCM-SS steel. During sliding, these carbides can delay the recovery process and postpone martensitic softening. The high temper stability postpones the transition from mild to severe wear and ensures that conditions of mild oxidative wear are maintained. Mild oxidative wear is the dominant wear mechanism for SDCM-SS steel between 400 and 700 °C.

Wear Resistance of H13 and a New Hot-Work Die Steel at High temperature

Effect of directional solidification of electroslag remelting on the microstructure and primary carbides in an austenitic hot-work die steel

The irreversible hydrogen traps including Cr-rich M7C3, Mo2C and MC carbides are characterized by scanning transmission electron microscope (STEM) after the quenching-and-tempering process performed on the API-5CT-C110 casing steel. Combined with our previously published work [Mater. Sci. Eng. A 688 (2017) 378–387], a systematic analysis with respect to the hydrogen interactions with reversible traps (grain boundaries and dislocations) and precipitates influencing sulfide stress cracking (SSC) behavior is presented. Grain boundaries provide channels for hydrogen diffusion. Dislocations trap hydrogen and act as hydrogen “transporters” during moving. Precipitates are introducing traps to retain hydrogen and restrict dislocation movement, then improve resistance to SSC. However, the mobile dislocations can transport the trapped hydrogen from precipitates into potential crack sites in the presence of external stress, leading to the initiation of SSC cracks. Therefore, SSC susceptibility significantly depends on the density of dislocations. The steel with lower dislocation density exhibits superior SSC behavior mainly attributed to its finite hydrogen “transporters”.

A systematical analysis with respect to multiple hydrogen traps influencing sulfide stress cracking behavior of API-5CT-C110 casing steel

Utilizing the hardness measurement, an attempt is made to study the tempering kinetics of DM hot-work die steels, combined with scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The results demonstrate that the increasing of tempering temperature will result in precipitation and coarsening of the intra-lath spherical carbides. Meanwhile, it is significantly observed the dissolution of needle-like Mo2C carbides with increasing of tempering temperatures. The growth mechanism of carbides is coarsening of large spheroids after complete and impingement precipitation on dislocations. In addition, the activation energy ΔH of DM steels during additional tempering between 620 °C and 700 °C is slightly higher than that of the diffusion of usual alloying elements (e.g. Cr, V, Mo and W) in ferrite, which may be attributed to a higher Mn content in DM steels that dissolves into carbides and retardsthe coarsening of carbides at high temperature.

Microstructure evolution and kinetic analysis of DM hot-work die steels during tempering

Dilatometry measurement, transmission electron microscope (TEM), and a three-dimensional atom probe (3DAP) are utilized to study how adding Si affects the kinetics of martensitic steel during tempering. The results show that both the tempering transformation temperature and the activation energy of martensitic hot-work die steel increase remarkably when Si is added. Meanwhile, the film-type retained austenite and the Si-rich region adjacent to the carbides are each observed in tempered martensitic steels. The results suggest that coarsening of carbides is efficiently inhibited and softening of steels is sufficiently delayed.

Qingchun Zhou

Papers

Microstructure evolution and kinetic analysis of DM hot-work die steels during tempering

Effect of Si addition on kinetics of martensitic hot-work die steel during tempering