Chemical composition and structural identification of primary carbides in as-cast H13 steel

The aim of this study was to characterize the primary carbides formed in as-cast H13 steel. The composition, morphology, type, and size of primary carbides in as-cast H13 steel were investigated by optical microscope (OM), field emission scanning electron microscopy (FE-SEM), electron back-scattered diffraction (EBSD), and X-ray diffraction (XRD) analysis. The number of primary carbides was investigated by ASPEX automated inclusion analysis system. The results indicated that primary carbides in as-cast H13 steel are mainly composed of Cr, Mo, V, and Ti, and there exist four kinds of primary carbides in the interdendritic zones of H13 steel, which are stripy Mo–Cr-rich M2C, eutectic Mo–Cr-rich M2C, V-rich MC, and V-rich MC with Ti and N. Thermodynamic calculation indicated that M2C precipitates in liquid phase at solid fractions larger than 0.99, while MC precipitates in liquid phase at solid fractions larger than 0.96. Statistical results indicated that the number of M2C is much greater than the number of other kinds of primary carbides. Most primary carbides are blocky, with lengths of no more than 10 μm and a length/width ratio of no more than 3. The large primary carbides in as-cast H13 steel are mainly M2C.

Microstructure and Properties of Low Temperature Composite Chromized Layer on H13 Tool Steel

Low temperature composite chromizing is a process composed of a plain ion-carbonitriding or ion-nitriding at 550-580℃, followed by a low-temperature chromizing in a salt-bath of 590℃. The microstructure and properties of the low temperature composite chromized layer on H13 tool steel were investigated using metallography, X-ray diffraction, microanalysis, hardness and wear tests. It was found that this low temperature process was thermo-dynamically and kinetically possible, and the composite chromized layer on H13 steel, with a thickness of 3-6 μm, consisted of three sub-layers (bands), viz. the outer Cr-rich one, the intermediate (black) one, and the inner, original white layer. After chromizing, the former diffusion layer was thickened. The results of X-ray diffraction showed that the composite chromized layer contained such nitrides and carbides of chromium as CrN, Cr2N, (Cr, Fe)23C6, and (Cr, Fe)7C3, as well as plain α-(Fe, Cr). A high surface microhardness of 1450-1550 HV0.025, which is much higher than that obtained by the conventional ion carbonitriding and ion nitriding, was obtained. In addition, an excellent wear resistance was gained on the composite chromized layer.

Study on Heterogeneous Nuclei in Cast H13 Steel Modified by Rare Earth

The dendrite segregation in cast H13 steel was weakened with RE modification treatment. Grain boundary carbide during quenching was also under control and impact toughness was improved greatly. By thermodynamic calculation, analysis of two-dimensional lattice misfitting and electron probe analysis, it is found that Ce2O3 may act as the heterogeneous nuclei of modified cast H13 steel.

Study on primary carbides precipitation in H13 tool steel regarding cooling rate during solidification

This study aims to investigate the primary carbides precipitation in H13 steel solidified at relatively high cooling rates,ranging from 300 to 6,000℃·min^-1,based on in situ observations with a high temperature confocal laser scanning microscope.In the cooling rate range investigated,the solidification microstructure becomes more refined as cooling rate increases and the relationship between the secondary dendrite arm spacing(SDAS),λ2,and cooling rate,.T,can be expressed asλ2=128.45.T-0.124.Regardless of cooling rates,two kinds of primary carbides,i.e.,the Mo-Cr-rich and V-rich carbides,are precipitated along the interdendritic region and most of them are the Mo-Cr-rich carbides.The morphology of Mo-Cr-rich carbide is not obviously influenced by the cooling rate,but that of V-rich carbide is obviously affected.The increasing cooling rate markedly refines the primary carbides and reduces their volume fractions,but their precipitations cannot be inhibited even when the cooling rate is increased to 6,000℃·min^-1.Besides,the segregation ratios(SRs)of the carbides forming elements are not obviously affected by the cooling rate.However,compared with the conventionally cast ingot,the SDAS and primary carbides in the steel solidified at the investigated cooling rates are much finer,morphologies of the carbides have changed significantly,and SRs of the carbides forming elements are markedly greater.The variation of primary carbide characteristics with cooling rate is mainly due to the change in SDAS.

Effect of Thermal Fatigue Loading on Tensile Behavior of H13 Die Steel with Biomimetic Surface

Biomimetic surface is an effective ways to promote the performance grade and applied range of materials without alteringtheir substrate.Many improved properties such as resisting fatigue,enduring wear,etc,have been achieved by applyingbiomimetic morphology or structure to some engineering material surfaces.In this paper,aiming to reveal the relationshipbetween thermal cracking behavior and mechanical properties of engineering materials with biomimetic surface,biomimeticspecimens were fabricated using laser technique by imitating the heterogeneous structure on the surface of plant leaves.Theeffect of thermal fatigue cycling on the tensile properties of H13 die steel specimens with different surfaces (several types ofbiomimetic surfaces and a smooth surface) was compared and investigated.As a result,due to the coupling effects of themorphological features on the surface and the microstructure characteristics within unit zone,these specimens with biomimeticsurface exhibit remarkably enhanced Ultimate Tensile Strength (UTS) and 0.2% Yield Strength (YS) compared with referencespecimens while corresponding ductility remains largely unaffected even heightened,whether the thermal fatigue loads or not.The relative mechanisms leading to these improvements have been discussed.

Effects of quenching temperature on microstructure and mechanical properties of H13 mandrel steel

Optical microscope, stereomicroscope, scanning electronic microscope （SEM） and mechanical property testing were used to research the effects of different quenching temperatures on the microstructure and mechanical properties of the H13 mandrel steel. The results indicate that following an increase in the quenching ,the degree of alloying is enhanced due to the carbides dissolving gradually in austenite, which improves the hardenability of the specimens, as well as their room and high-temperature strength. At the same time, the fracture toughness increases due to the increment of the martensite number and the interparticle distance of impact toughness. Optimal performance Consequently,the service life of the H13 the carbides. However, extremely coarse grain and martensite can decrease the can be obtained after quenching at 1 060℃ and double tempering at 620℃mandrel steel is extended significantly.

Influence of Surface Nano-structured Treatment on Pack Boriding of H13 Steel

In order to lower the boriding temperature of hot work steel H13, method of surface mechanical attrition treatment （SMAT）, which can make the grain size of the surface reach nano-scale, was used before pack boriding. The growth of the boride layer was studied in a function of boriding temperature and time. By TEM （transmission electron microscopy）, SEM （scanning electron microscopy）, XRD （x-ray diffraction） and microhardness tests, the grain size, thermal stability of the nano-structured （NS） surface and the thickness,appearance, phases of the surface boride layer were studied. Kinetic of boriding was compared between untreated samples and treated samples. Results showed that after SMAT, the boride layer was thicker and the hardness gradient was smoother. Furthermore, after boriding at a low temperature of 700℃ for 8 h, a boride layer of about 5 μm formed on the NS surface. This layer was toothlike and wedged into the substrate, which made the surface layer combine well with the substrate. The phase of the boride layer was Fe2B. Research on boriding kinetics indicated that the activation energy was decreased for the treated samples.

Development of Flow Stress of AISI H13 Die Steel in Hard Machining

An approach was presented to characterize the stress response of workpiece in hard machining, accounting for the effect of the initial workpiece hardness in addition to temperature, strain and strain rate on flow stress in this paper. AISI H13 die steel was chosen to verify this methodology. The proposed flow stress model demonstrates a good agreement with experimental data. Therefore, the proposed model can be used to predict the corresponding flow stress-strain response ofAISl H13 die steel with variation of the initial workpiece hardness in hard machining.

Laser surface hardening of AISI H13 tool steel

An attempt was made to improve the surface hardness and wear properties of AISI H13 tool steel through solid solution hardening and refinement of microstructures using a 200 W fiber laser as a heat generating source.The hardness of laser melted zone was investigated.In order to identify the effect of heat input on the laser melting zone,scanning conditions were controlled.The results show that,the hardness of as-received AISI H13 tool steel is approximately Hv 240,and the hardness after laser surface heat treatment is around Hv 480-510.The hardening depth and width are increased with the increase in the heat input applied.Application of experimental results will be considered in tooling industry.

Simulations of Coarsening Behavior for M_(23)C_6 Carbides in AISI H13 Steel

Based on the local equilibrium assumption, coarsening behavior of M23C6 carbide at 700℃ in H13 steel was simulated by DICTRA software. The results from the calculations were compared with transmission electron microscopy （TEM） observations. The results show the interracial energy for M23C6 in H13 steel at 700℃ is thus probably 0.7J·m^-2, which fits the experiments well. The influence of composition and temperature on the coarsening rate was also investigated by simulations. Simulations show a decrease in the coarsening rate when V/Mo ratio is increased, while the coarsening rate increases with increasing temperature.

Deoxidation of H13 tool steel with CaF_(2)-MgO-CaO-Al_(2)O_(3)-SiO_(2) slags at 1873 K

Laboratory-scale experiments were performed to investigate the deoxidation of H13 tool steel with CaF_(2)-MgO-Al_(2)O_(3)-CaO-SiO_(2) slags at 1873 K.The calculation of thermodynamics and kinetics was also verified through the experimental results.The results show that[Si]-[O]reaction is the control reaction,and with the increase of basicity of slag,the limitation of deoxidation was decreased.The limitation of deoxidation is the lowest for the slag with basicity of 2.0.Under the conditions of the basicity of 2.0 and the content of CaF_(2) more than 50%,the limitation of deoxidation is less than 10×10^(−6),and it does not depend on the contents of Al_(2)O_(3) and CaF_(2) in slags.The mass transport of oxygen in the metal phase is the rate-controlling step,and the slag composition has no effect on the equilibrium time of deoxidation.Based on this finding,the optimized slag composition is designed and it contains the following components:51.5%CaF_(2),20.3%MgO,16.2%Al_(2)O_(3),8.2%CaO and 3.8%SiO_(2).In the case of the optimized deoxidizing slag,the total oxygen content in H13 steel can be reduced from 25×10^(−6) to 6×10^(−6).

Selective laser melting of Fe-Ni-Cr layer on AISI H13 tool steel

An attempt to fabricate Fe-Ni-Cr coating on AISI H13 tool steel was performed with selective laser melting.Fe-Ni-Cr coating was produced by experimental facilities consisting of a 200 W fiber laser which can be focused to 80 μm and atmospheric chamber which can control atmospheric pressure with N2 or Ar.Coating layer was fabricated with various process parameters such as laser power,scan rate and fill spacing.Surface quality and coating thickness were measured and analyzed.Three different surface patterns,such as typeⅠ,typeⅡand type Ⅲ,are shown with various test conditions and smooth regular pattern is obtained under the conditions as 10 μm of fill spacing,50-350 mm/s of scan rate and 40 μm of fill spacing,10-150 mm/s of scan rate.The maximum coating thickness is increased with power elevation or scan rate drop,and average thickness of 10 μm fill spacing is lower than that of 40 μm fill spacing.

Microstructures and properties of Co-based alloy coatings prepared on surface of H13 steel

The Co-based alloy coatings had been prepared by laser cladding and vacuum fusion sintering. Microstructures of the coatings were investigated and the performance of thermal cycling was also tested using scanning electron microscopy （ SEM） and X-ray diffraction （ XRD ）. The results show that the coatings and substrates combine well. The main phase compositions of laser cladding coating are T-Co, Cr23 C6 and Ni2 9 Cro. 7 Feo. 36, while vacuum fusion sintering coating consists of Co, Cr7 C3, and Ni2.9 Cro. 7 Feo. 36. After thermal cycling, the minimum hot cracking width of laser cladding coating is 14 μm; moreover, laser cladding coating maintains high hardness and hot-cracking susceptibility. Those are beneficial to high temperature wear resistance of hot work dies.

Thermal fatigue behavior of niobium microalloyed H13 steel

AISI H13 hot work tool steel is widely used for hot forging, hot-extrusion and die-casting because of its high temperature strength, impact toughness, heat checking resistance and wear resistance, etc. The thermally induced surface damage, i. e., thermal fatigne,is believed to be controlled by the magnitude of the imposed cyclic strain. The thermal fatigue on the surface of hot working die, which is responsible to the initiation of the cracks, is reported to result in more than 80 % of the failure of dies.

FLOW STRESS MODEL FOR HARD MACHINING OF AISI H13 WORK TOOL STEEL

An approach is presented to characterize the stress response of workpiece in hard machining, accounted for the effect of the initial workpiece hardness, temperature, strain and strain rate on flow stress. AISI H13 work tool steel was chosen to verify this methodology. The proposed flow stress model demonstrates a good agreement with data collected from published experiments. Therefore, the proposed model can be used to predict the corresponding flow stress-strain response of AISI H13 work tool steel with variation of the initial workpiece hardness in hard machining.

Study of the influence of heat treatment procedure on the microstructure and mechanical properties of H13 mandrel steel

In this study,the microstructural evolution of H13 mandrel steel during its manufacturing process and the influence of the preheat treatment procedure and the second-step tempering temperature on its microstructural and mechanical properties were investigated.The experiment results showed that,using an H13 mandrel steel billet in the as-annealed condition produced by Baosteel as a raw material,the preheat treatments of normalization,spheroidization annealing,or normalization plus spheroidization annealing,as described in this paper,had no significant effect on the final mechanical properties after quenching and tempering.H13 mandrel steel met all the mechanical property requirements via quenching and tempering without preheat treatment.In the temperature range from 600 to 680 t,with an increase in the second-step tempering temperature,the strength gradually decreased whereas the plasticity and toughness gradually improved.Using the quenching-plus-first-tempering procedure of 1030℃/l h/oil cooling+600℃/5 h/air cooling,good balance between strength and toughness could be achieved by selecting 650℃ as the second-step tempering temperature.The test results also showed that the holding time of the second tempering played a big role in the mechanical properties.When this holding time was extended from 1 to 2 h,the hardness of the H13 mandrel steel decreased and the extent of this reduction increased with an increase in the second-step tempering temperature.In the temperature range of 1030 to 1090℃,the yield strength,tensile strength,and hardness gradually increased,however,the impact toughness decreased with increases in the quenching temperature.

The Effect of Heat Treatment Atmosphere on Hardening of Surface Region of H13 Tool Steel

The main objective of the die heat treatment is to enhance the surface hardness and wear properties to extend the die service life. In this paper, a series of heat treatment experiments were conducted under different atmospheric conditions and length of treatment. Four austenitization atmospheric conditions were studied and although each heat treatment condition resulted in a different hardness profile, it did not affect the results for gas nitriding. All samples subjected to the nitriding process produced similar thicknesses of hardened case layer with average hardness of 70 - 72 HRC if the initial carbon content is not too low. It was shown that heat treatment without atmospheric control results in a lower hardness on the surface since the material was subjected to decarburization effect. The stainless steel foil wrapping around the sample and heat treatment in a vacuum furnace could restrict the decarburization process, while pack carburization heat treatment resulted in a carburization effect on the material.

Influence of Deep Cryogenic Treatment (DCT) on Thermo Mechanical Performance of AISI H13 Tool Steel

Deep cryogenic treatment (DCT) is one of the most recent processes being used to treat tool steels. It is a supplementary treatment where components are treated below subzero temperatures for several cryo-soaking hours. This paper focuses on to study the effect of deep cryogenic treatment on Thermo mechanical properties of AISI H-13 tool steel. Deep cryogenic treatment at 32 hours of cryo-soaking time is applied and thermo mechanical performance of tool steel was analyzed by using pin on disk high temperature wear testing and stress rupture testings. The microstructural evolutions during DCT were evaluated by using scanning electron microscope (SEM). It was observed that microstructural modifications like increase in carbide density, fine and uniform martensitic structure during DCT had significantly improved properties.

Effects of carbon,sulphur,phosphorus,and nickel compositions and hot working on thermal fatigue properties of H13 steel

For precise composition control and the selection of hot-w orking operation,effects of carbon（ C）,sulphur（ S）,phosphorus（ P）,and nickel（ Ni） compositions and hot working on thermal fatigue properties of H13 steel under thermal cycling conditions w ere studied by a self-restraint thermal fatigue test. Results show ed that the control of the C content at about 0. 35%,Ni content at about 0. 50%,and S and P contents as low as possible and the forging operation used lead to an improvement of the thermal fatigue resistance of the steel.

热处理对H13热作模具钢的显微结构和性能的影响

研究预备热处理工艺、回火温度及深冷处理对H13热作模具钢的显微结构和力学性能的影响。结果表明,对成分偏析且锻造不足的H13钢,在常规的热处理条件下,适当提高回火温度(如630℃)可部分消除H13钢的带状组织;对该种钢材热处理前进行扩散退火+球化退火预先热处理,能更有效改善其金相组织,较大幅度提高材料的冲击韧度;深冷处理后,残余奥氏体转变为马氏体,同时碳化物分布更加细小、均匀,可进一步提高H13钢的力学性能。