The effect of heat treatment on microstructure and mechanical properties (microhardness, wear resistance and impact toughness) of laser additively manufactured AISI H13 tool steel was systemically investigated. To understand the variation of microstructure and mechanical properties under different heat treatments, the as-deposited samples were treated at 350, 450, 550, 600 and 650 °C/2 h, respectively. Microstructure and phase transformation were investigated through optical microscopy, scanning electron microscope and transmission electron microscope. The mechanical properties were characterized by nanoindentation tests, Charpy tests and high-temperature wear tests. The microstructure of as-deposited samples consisted of martensite, ultrafine carbides and retained austenite. After the tempering treatment, the martensite was converted into tempered martensite and some fine alloy carbides which precipitated in the matrix. When treated at 550 °C, the greatest hardness and nanohardness were 600 HV0.3 and 6119.4 MPa due to many needle-like carbides precipitation. The value of hardness increased firstly and then decreased when increasing the temperature. When tempered temperatures exceeded 550 °C, the carbides became coarse, and martensitic matrix recrystallized at the temperature of 650 °C. The least impact energy was 6.0 J at a temperature of 550 °C. Samples tempered at 550 °C had larger wear volume loss than that of others. Wear resistances of all samples under atmospheric condition at 400 °C showed an oxidation mechanism.

Effect of Heat Treatment on Microstructure and Mechanical Properties of Laser Additively Manufactured AISI H13 Tool Steel

Impurity and alloying elements on grain surface in iron: Periodic dependence of binding energy on atomic number and influence on wear resistance

The ductility drop and decrease in strength that lead to crack formation during continuous casting of steel is typically investigated by means of the hot ductility test. In this study, hot ductility tests are performed by using a thermo-mechanical Gleeble system to simulate the deformation of steels at high temperatures and low deformation rates similar to those during continuous casting. Thus, temperature was varied between 600 and 1000 °C while strain rates covered a range from 0.001 to 0.1 s − 1 . Tests are carried out to identify the temperature range at which the steel is susceptible to crack formation as well as the effect of strain rate. Characterization of fractured surfaces and phase transformation after thermo-mechanical tests are conducted in the SEM and Optical Microscope. The combination of these techniques makes possible to formulate cracking mechanisms during hot processing which show critical strain for failure at temperatures between 700 and 900 °C based on the convergence of three different criteria: I) Reduction of area, II) True fracture strength-ductility and III) True total energy. This approach provides a better understanding of crack formation in steels at the high temperatures experienced during continuous casting. This information is key to productivity losses and avoid defect formation in the final cast products.

Converging criteria to characterize crack susceptibility in a micro-alloyed steel during continuous casting

There are many types of non-metallic MgAl2O4 inclusions observed in Al-deoxidized steel coupling with Mg treatment, including single-particle MgAl2O4, agglomerated MgAl2O4, and MgAl2O4-MnS. Thermodynamic calculation shows that MgAl2O4 precipitates in the liquid phase. The phase transformation follows liquid + Al2O3 + MgAl2O4 → liquid + MgAl2O4 → liquid + MgO + MgAl2O4 → liquid + MgO with the Mg content increasing when the Al content is a constant in molten steel, and it is in agreement with the experimental results for the formation of MgAl2O4 in molten steel. The calculation results of various attractive forces between two particles show that the cavity bridge force plays a dominant role in the agglomeration process and results in the agglomerated MgAl2O4. The lattice mismatch calculation result shows that MgAl2O4 can provide effective sites for MnS nucleating in steel.

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Thermodynamics and Agglomeration Behavior on Spinel Inclusion in Al-Deoxidized Steel Coupling with Mg Treatment

The paper presents the results of tests of a welded joint of Hardox 450 steel, belonging to the group of weldable high-strength boron steels with increased resistance to abrasive wear. As a result of the conducted research, apart from the basic structural indicators, an attempt was made to determine the correlation between the grain size of the prior austenite in the characteristic weld zones and its basic mechanical properties, such as yield point, tensile strength, percentage elongation after fracture, reduction of area, and impact strength. The scope of research quoted above was carried out for a welded joint of the considered steel at delivery state (directly after welding), in the normalising annealed state, as well as in water-quenched state, using different austenitisation temperatures in the range of 900-1200 °C. The results obtained showed a large influence of the parameters of the applied thermal heat treatment on the selected structural and mechanical properties of the welded joint.

Austenite Grain Growth Analysis in a Welded Joint of High-Strength Martensitic Abrasion-Resistant Steel Hardox 450.

The effect of boron addition on strengthening of steel Q690D was studied. The metallographic microstructures of Q690D plate samples after controlled rolling and cooling process and roughing rolling in bench-scale were observed by optical microscope and transmission electron microscope, and the mechanical properties were determined. The results showed that the microstructure of Q690D became finer distinctly with increase in the content of soluble boron in steel. The yield strength and tensile strength also increased with the increase of soluble boron. Most of boron in steel Q690D existed in solid solution state. The main precipitates in Q690D steel were the Ti(C, N) and NbC. The amount of BN was small. Among the strengthening performance of steel Q690D given by boron addition, refinement strengthening gave the greatest contribution on the strengthening effect.

Effect of Micro-alloying Element Boron on the Strengthening of High-Strength Steel Q690D

Hot tensile tests were performed to examine the effect of boron on the hot ductility of Nb-Ti-microalloyed steels. The equilibrium precipitation in the steel was predicted by Thermo-Calc calculation. The microstructure, fracture surface and precipitates in the deformed steel were examined. The results show that boron addition is favorable to improving the hot ductility of Nb-Ti-microalloyed steel. This bene cial effect is caused by the soluble boron instead of coarse BN in the steel. The hot ductility of the steel decreases less from 1000°C with increasing boron addition. The hot ductility trough shifts toward lower temperatures because ferrite formation was restrained with increasing boron content of the steel. The formation of NbC, TiN and thin lm-like ferrite along austenite grain boundaries lead to the decrease in the hot ductility of the steel. Boron addition has negligible in uence on the precipitation temperature and amount of TiN and NbC precipitates in Nb-Ti-microalloyed steel. The amount of NbC precipitates is largest in the steel, followed by TiN and BN. The precipitation temperature of BN increases considerably with further increasing the boron content. The fracture mode of Nb-Ti-microalloyed steel tends to be more ductile with the increase in the boron content of the steel. [doi:10.2320/matertrans.M2015388]

Effect of Boron on the Hot Ductility of Low-Carbon Nb-Ti-Microalloyed Steel

Effects of magnesium on wear resistance of H13 steel have been investigated. A disc-on-disc configuration with load of 200N was employed to study the wear behavior. In order to understand wear mechanisms, wear tracks were studied by scanning electron microscopy. The experimental results show that after magnesium treatment finer carbides were precipitated and the hardness increased with the increasing of magnesium content, which can improve the wear resistance. The specimen without magnesium treatment has the lowest wear resistance and the mode of wear is adhesive. With increase of magnesium content, the degree of oxide patches coverage increased result in the wear rate decreased gradually. Wear mechanism has been changed from adhesive to oxidative after the magnesium content increased to 10 ppm. In this case, the friction coefficient exhibits a higher fluctuation resulted from the alternative formation and delamination of oxide layers. [doi:10.2320/matertrans.M2014086]

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Effects of Magnesium on Wear Resistance of H13 Steel

The hot ductility of low-carbon aluminum-killed steel with different boron contents was investigated. The hot tensile experiment was conducted using Gleeble3500 simulator. The reduction of area of each specimen was determined to estimate the quality of hot ductility. The changes of precipitates in studied steel were calculated by Thermo-Calc software. The microstructures of specimens were observed using optical microscopy (OM), sanning electron microscopy (SEM) and transmission electron microscopy (TEM). Based on the experimental results, the fracture types of specimens were analyzed. The results show that the hot ductility improves with increase in boron content. In the experimental condition of this study, the combining ability of boron with nitrogen is greater than that aluminum and nitrogen in the energetics point of view. Coarse BN particles decrease the effect of pinning grain boundary by fine AlN. Boron in solid solution plays a role of inhibiting the austenite/ ferrite transformation. The improvement of hot ductility is caused by the formation of coarse BN and existence of boron in solid solution. [doi:10.2320/matertrans.M2015069]

Influence of Boron Addition on the Hot Ductility of Low-Carbon Aluminum-Killed Steel

Based on the generalized flexibility matrix, a method for detecting structural damage is presented in this paper. The generalized flexibility matrix is approximately constructed by using the first frequency and the corresponding mode shape only. Then the difference of generalized flexibility curvature between undamaged and damaged state is used to detect the possible damaged elements. Finally, a numerical example concerning a simple supported beam is used to illustrate the effectiveness of the proposed method.

Jing Li

Papers

Effect of Micro-alloying Element Boron on the Strengthening of High-Strength Steel Q690D

Effect of Boron on the Hot Ductility of Low-Carbon Nb-Ti-Microalloyed Steel

Effects of Magnesium on Wear Resistance of H13 Steel

Influence of Boron Addition on the Hot Ductility of Low-Carbon Aluminum-Killed Steel

Structural Damage Detection Based on Generalized Flexibility Matrix