Papers by Keyword: Hardenability

Abstract: The effects of Mo and V on impact toughness in martensitic steels tempered at low temperatures were investigated using three low-alloy medium-C steels. Previous examination of these alloys had identified differences in impact toughness without a clear cause. In this work, the Base alloy with a reduced Mo addition experienced a significant loss in hardenability leading to the formation of small fractions of bainite during quenching even at relatively high quench rates. The use of different quench media to simulate cooling rates throughout a heavy section demonstrated that the variation in previously reported Charpy V-notch impact absorbed energies was readily explained by some regions cooling fast enough to avoid bainite while others formed some small fraction of upper bainite leading to increased cleavage fracture and decreased impact toughness. Small amounts of bainite transformation were not detected by dilatometry or tensile properties. These results emphasize the importance of effective through-hardening and careful microstructure evaluation in alloys that are meant to maintain good toughness and strength in thicker sections.

Abstract: The press hardening steel with thick specification has been used as automobile chassis parts. With the increase of the thickness, the cooling rate of the inner core is lower in the conventional process, resulting in a poor harden ability, and the fatigue test is not qualified. In this study, the microstructure of hot forming parts with thick specifications was studied by means of metallographic and micro-hardness testing. The results show that there is a microstructure gradient in the thick hot forming parts, which leads to the uneven strength in the cross section and finally affects the fatigue test results.The finite element analysis method is used to study the cooling rate of the inner core of the hot forming parts. The high hardenability press hardening steel products has been prepared by adjusting the components.

Abstract: This study investigates the influence of the substitution of chromium (-0.8 wt.−%) by manganese (+1.3 wt.−%) in a standard quench and tempering steel and the predictability of property changes through simple simulations, only dependent on the chemical composition. The substitution of chromium by manganese leads to an increased hardness (+50 HV10) and a reduction of the critical cooling speed from 19 K s−1 for the reference alloys to 9 K s−1 for the new alloy and a nearly constant hardness of (600 HV10) after Jominy-test. The commercial software JMATPRO is used to simulate and predict key properties for the industrial production. It is shown that a successful simulation of phase transformation temperatures and the general directions of change can be predicted, but more complex properties like critical cooling rates or hardenability need more sophisticated methods.

Abstract: The properties of steel 42HMFA, which has been tempered in various environments at thermal improvement, are investigated. Advantages of application to training of water solutions of polymers are shown. Training in synthetic environments provides higher rates of mechanical properties of steel products. As a result of training in water solutions of polymer, the maximum level of hardenability on the section of product and fatigue strength of detail is reached.

Abstract: The addition of titanium is a well-known microalloying concept for hot rolled structural steels. Concerning advanced high strength steels for the automotive sector, the use of Ti microalloying (usually with Nb-V) has been an active research area. However, Ti addition has not been explored in depth. For the current contribution, a laboratory hot rolled 0.2C-2.4Mn-1.5Si steel with and without Ti addition was studied. Mechanical testing of the hot strip revealed a very high UTS (1GPa) for the Ti added steel, whilst for the unalloyed chemistry the UTS was some 300 MPa lower. Observation of the hot rolled microstructures via optical microscopy showed a significantly higher hardenability for the Ti added steel. Moreover, X-ray diffraction analysis indicated a significant amount of retained austenite in the Ti added strip, which transformed completely to martensite after the tensile test. Further analysis via TEM and chemical extraction indicated that Ti was present both as Ti (C,N) precipitates and in solution. Finally, in light of these observations, the possible mechanisms leading to the enhanced hardenability observed for the Ti added hot rolled strip steel were discussed.

Abstract: Development of S6 tool steel has been discussed in this research by conducting a bit modification in the chemical composition using aluminum instead of molybdenum, and micro addition of boron to enhance the marteniste structure. Then, the hardenability and thermal stability have been detected in regarding to S6 tool steel. A novel isothermal treatment process has been suggested to enrich the retained austenite, and thereby, it has been tracked by using XRD, optical microscope, and SEM in conjugation with EDS. The effect of retained on the mechanical properties has been determined. The results ensure that aluminum has inhibited the graphite formation through the slow cooling regime. No change of hardenability or thermal stability of S6 tool steel triggered from adding of aluminum and microaddition of boron. In addition, the novel isothermal process leads to enrichment of retained austenite that has significantly affected on the combination of ductility with strength of the newly designed steel.

Abstract: Microalloying has received increased attention in recent years. The aims of the study were to identify and to examine the influence of nitrogen concentration in steel and small additions of nitride-forming elements on the hardenability of a boron-treated manganese-vanadium steel 40GF-VI. The study has shown that the increase in nitrogen concentration from 0.004% to 0.015% increases the hardenability of steel. It was found that a small amount of titanium (about 0.02%) in steel with low concentration of nitrogen (0.004%) is sufficient to bind it to nitrides, which makes it possible to save the most of boron in an active state (in a solid solution). A residual amount of titanium and aluminum in the range of 0.015-0.020% of each in steel with nitrogen concentration in the range of 0.010-0.015%, which is typical of an electric arc melting steel, is insufficient to bind all nitrogen. As a result, a part of nitrogen is spent on the formation of boron nitrides, which reduces the effect of boron on the hardenability of manganese-vanadium steel microalloyed with boron. Some methods of protecting boron in steel are briefly described. The study has established that grain refinement is observed with increasing nitrogen content in steel. Introduction of boron, in absence of titanium, does not change the size of the austenite grain in the entire range of the investigated temperatures. The optimum combination of strength, plastic and ductility characteristics in steel microalloyed with boron and additives of aluminum and titanium was obtained.

Abstract: Recent interests in developing novel super-high strength steels have led to extensive research efforts in direct quenching with or without tempering (DQ, DQT) or combined with partitioning (DQP). Both strip and plate products have been targeted for different applications. For boron-microalloyed DQ/DQT steels, the ASTM A255 approach for predicting the hardenability was considered inapplicable. Fresh attempts were made to develop new hardenability models through non-linear regression analysis by dynamically varying both the boron factor and multiplying factors of most elements in the alloy factor. Based on the recent concept of quenching and partitioning (Q&P), a novel processing route comprising thermomechanical rolling followed by direct quenching and partitioning (TMR-DQP) has been established for the development of ultra-high strength structural steels with yield strengths ≈1100 MPa combined with good uniform and total elongations and impact toughness. Examples of recent advances made in DQ processing and associated challenges, such as those related to the bendability of low carbon martensitic-bainitic steels and influence of boron on the toughness of Nb-bearing martensitic steels are presented.

Abstract: To fulfill the industrial demand of forged steels with high mechanical and microstructural requirements coupled with reduced cost, the possibility to decrease the content of Mo and other elements has been evaluated. In order to do that, the effect of boron addition (up to 30 ppm) on the steel hardenability has been investigated on two steels with different chemical composition at laboratory scale. In particular, the steel chemical composition has been designed in order to make effective the B addition in terms of hardenability. Two 80 kg ingots cast by a vacuum induction melting plant have been hot rolled by a pilot mill. The effect of B addition on hardenability has been evaluated and compared to that of steel for same application but without B. Results show an improvement of hardenability if 30 ppm B are added even if a Mo reduction is performed.

Abstract: The effects of austenite grain size on hardenability and impact toughness were investigated. The results show that: Since the beginning of the two-phase region with quenching temperature, the austenite grain size from the initial 4+6 mixed crystal at 740°C, and gradually increased to 10 at 860°C; Austenite grain size and hardenability was directly proportional to the austenite grain size increased from 8μm to 36μm, the biggest change is the hardness 10HRC; Austenite grain size and impact toughness is linear, with the decrease of grain size, the impact energy increases linearly, and the austenite grain size and impact toughness curve fitting. Comprehensive analysis for ensuring the hardenability of cold heading steels should be considered optimal matching of material strength and plasticity.