The effect of solidification cooling rate on the size and distribution of inclusions in 12%Cr stainless steel was investigated. A wide range of solidification cooling rates（from 0.05 to 106 K·s^-1） was achieved...The effect of solidification cooling rate on the size and distribution of inclusions in 12%Cr stainless steel was investigated. A wide range of solidification cooling rates（from 0.05 to 106 K·s^-1） was achieved using various solidification processes, including conventional casting, laser remelting, and melt spinning. The size and distribution of inclusions in the steel were observed and statistically collected. For comparison, mathematical models were used to calculate the sizes of inclusions at different solidification cooling rates. Both the statistical size determined from observations and that predicted from calculations tended to decrease with increasing cooling rate; however, the experimental and calculated results did not agree well with each other at excessively high or low cooling rate. The reasons for this discrepancy were theoretically analyzed. For the size distribution of inclusions, the effect of cooling rate on the number densities of large-sized（〉 2 μm） inclusions and small-sized（≤ 2 μm） inclusions were distinct. The number density of inclusions larger than 1 μm was not affected when the cooing rate was less than or equal to 6 K·s^-1 because inclusion precipitation was suppressed by the increased cooling rate.展开更多
In this paper, ultrasonic （20 kHz） fatigue tests were performed on specimens of a high-strength steel in very high cycle fatigue （VHCF） regime. Experimental results showed that for most tested specimens failed in ...In this paper, ultrasonic （20 kHz） fatigue tests were performed on specimens of a high-strength steel in very high cycle fatigue （VHCF） regime. Experimental results showed that for most tested specimens failed in a VHCF regime, a fatigue crack originated from the interior of specimen with a fish-eye pattern, which contained a fine granular area （FGA） centered by an inclusion as the crack origin. Then, a two-parameter model is proposed to predict the fatigue life of high-strength steels with fish-eye mode failure in a VHCF regime, which takes into account the inclusion size and the FGA size. The model was verified by the data of present experiments and those in the literature. Furthermore, an analytic formula was obtained for estimating the equivalent crack growth rate within the FGA. The results also indicated that the stress intensity factor range at the front of the FGA varies within a small range, which is irrespective of stress amplitude and fatigue life.展开更多
Fatigue properties of high-strength steels become more and more sensitive to inclusions with enhancing the ultimate tensile strength (UTS) because the inclusions often cause a relatively low fatigue strength and a lar...Fatigue properties of high-strength steels become more and more sensitive to inclusions with enhancing the ultimate tensile strength (UTS) because the inclusions often cause a relatively low fatigue strength and a large scatter of fatigue lives. In this work, four S–N curves and more than 200 fatigue fracture morphologies were comprehensively investigated with a special focus on the size and type of inclusions at the fatigue cracking origin in GCr15 steel with a wide strength range by different heat treatments after high-cycle fatigue (HCF). It is found that the percentage of fatigue failure induced by the inclusion including Al2 O3 and TiN gradually increases with increasing the UTS, while the percentage of failure at sample surfaces decreases conversely and the fatigue strength first increases and then decreases. Besides, it is interestingly noted that the inclusion sizes at the cracking origin for TiN are smaller than that for Al2 O3 because the stress concentration factor for TiN is larger than that for Al2 O3 based on the finite element simulation. For the first time, a new fatigue cracking criterion including the isometric inclusion size line in the strength-toughness coordinate system with specific physical meaning was established to reveal the relationship among the UTS, fracture toughness, and the critical inclusion size considering different types of inclusions based on the fracture mechanics. And the critical inclusion size of Al2 O3 is about 1.33 times of TiN. The fatigue cracking criterion could be used to judge whether fatigue fracture occurred at inclusions or not and provides a theoretical basis for controlling the scale of different inclusion types for high-strength steels. Our work may offer a new perspective on the critical inclusion size in terms of the inclusion types, which is of scientific interest and has great merit to industrial metallurgical control for anti-fatigue design.展开更多
The change of inclusions and microstructure of 16Mn steel treated by Ce were observed,and the effect of austenitizing temperature on the microstructure was also examined.The results show that the inclusions are transf...The change of inclusions and microstructure of 16Mn steel treated by Ce were observed,and the effect of austenitizing temperature on the microstructure was also examined.The results show that the inclusions are transformed from Si-Mn-Al composite oxide and MnS into AlCeO3,Ce2O2S,and MnS composite inclusions after being treated by Ce.Plenty of intragranular ferrites are formed in 16Mn steel containing～0.017wt% Ce.A large amount of intragranular acicular ferrites are formed after being austenitized for 20min at 1473 K.The prior austenite grain size fit for the formation of intragranular acicular ferrites is about 120μm.展开更多
Speed increase and heavy haul of the railway freight car are the effective measures to raise the transport ability of railway, and they have been the new trend for railway freight car development. The increase of spee...Speed increase and heavy haul of the railway freight car are the effective measures to raise the transport ability of railway, and they have been the new trend for railway freight car development. The increase of speed and axle load will lead to the increase of wheel rail contact stress, which will make a great change of stress distribution in the whole wheel rim. Goodier equation was employed to analyze the stress state around inclusion and cavity. In the operation of wheel-rail contact stress, stress concentration occurred in the pole of Al2O3 spherical inclusion, while it appeared on the equator of cavity. The critical inclusion sizes of 250 kN axle load freight car wheel at different velocities, a certain distance away from the tread, were calculated by Murakami equation. Comparing the inclusion in cast steel wheel with that in rolled steel wheel, it is found that the inclusion in cast steel wheel has much more excellent crack resistance than that in rolled steel wheel. In the meantime, in order to reduce the probability of rim crack, some suggestions were put forward.展开更多
The realization of an ideal combination of mechanical and fatigue properties is prerequisites for practical application of titanium(Ti)microalloyed steel in automotive field.The fatigue behavior of four Ti microalloye...The realization of an ideal combination of mechanical and fatigue properties is prerequisites for practical application of titanium(Ti)microalloyed steel in automotive field.The fatigue behavior of four Ti microalloyed high-strength beam steels with different Ti contents was systematically studied.The results show that the content of microalloying element Ti has a significant effect on the fatigue properties,especially in the steel with a high Ti content.For the experimental Ti microalloyed steel,inclusion-induced crack initiation is the main fatigue failure mode.Different from general fatigue fracture mechanism in Ti-contained steel,no TiN,which is the most detrimental to fatigue behavior,was found in fatigue crack initiation area.However,the large-sized TiN and oxide complex inclusion with a core-shell structure is the dominant cause of fatigue fracture.Because of the intense-localized deformation at the interface between complex inclusion and matrix,the angular TiN in the outer shell has a serious deteriorating effect on the fatigue properties,which is consistent with the result of the Kernel average misorientation map.Besides,the modification effect of a small amount of MnS on large-sized inclusion is not obvious and has little effect on the fatigue behavior.For more practical guidance,the critical inclusion sizes of the experimental steels were also investigated by experimental extrapolation method.With the increasing tensile strength,the inclusion sensitivity of the experimental steels increases,leading to the small critical inclusion size.展开更多
基金financially supported by the National Basic Research Program of China (No. 2011CB012900)
文摘The effect of solidification cooling rate on the size and distribution of inclusions in 12%Cr stainless steel was investigated. A wide range of solidification cooling rates（from 0.05 to 106 K·s^-1） was achieved using various solidification processes, including conventional casting, laser remelting, and melt spinning. The size and distribution of inclusions in the steel were observed and statistically collected. For comparison, mathematical models were used to calculate the sizes of inclusions at different solidification cooling rates. Both the statistical size determined from observations and that predicted from calculations tended to decrease with increasing cooling rate; however, the experimental and calculated results did not agree well with each other at excessively high or low cooling rate. The reasons for this discrepancy were theoretically analyzed. For the size distribution of inclusions, the effect of cooling rate on the number densities of large-sized（〉 2 μm） inclusions and small-sized（≤ 2 μm） inclusions were distinct. The number density of inclusions larger than 1 μm was not affected when the cooing rate was less than or equal to 6 K·s^-1 because inclusion precipitation was suppressed by the increased cooling rate.
基金supported by the National Basic Research Program of China (Grant 2012CB937500)the National Natural Science Foundations of China (Grants 11172304 and 11202210)
文摘In this paper, ultrasonic （20 kHz） fatigue tests were performed on specimens of a high-strength steel in very high cycle fatigue （VHCF） regime. Experimental results showed that for most tested specimens failed in a VHCF regime, a fatigue crack originated from the interior of specimen with a fish-eye pattern, which contained a fine granular area （FGA） centered by an inclusion as the crack origin. Then, a two-parameter model is proposed to predict the fatigue life of high-strength steels with fish-eye mode failure in a VHCF regime, which takes into account the inclusion size and the FGA size. The model was verified by the data of present experiments and those in the literature. Furthermore, an analytic formula was obtained for estimating the equivalent crack growth rate within the FGA. The results also indicated that the stress intensity factor range at the front of the FGA varies within a small range, which is irrespective of stress amplitude and fatigue life.
基金financially sup-ported by the National Natural Science Foundation of China(NSFC)(Grant Nos.52001310,52130002,and 51771208)the Strategic Pri-ority Research Program of the Chinese Academy of Sciences(Grant No.XDC04040502)+3 种基金the National Science and Technology Major Project(No.J2019-VI-0019-0134)Outstanding Postgraduate Inno-vative Research Project of Institute of Metal Research,CAS(No.1193002090)KC Wong Education Foundation(No.GJTD-2020-09)Institute of Metal Research Innovation Fund(No.2023-ZD01).
文摘Fatigue properties of high-strength steels become more and more sensitive to inclusions with enhancing the ultimate tensile strength (UTS) because the inclusions often cause a relatively low fatigue strength and a large scatter of fatigue lives. In this work, four S–N curves and more than 200 fatigue fracture morphologies were comprehensively investigated with a special focus on the size and type of inclusions at the fatigue cracking origin in GCr15 steel with a wide strength range by different heat treatments after high-cycle fatigue (HCF). It is found that the percentage of fatigue failure induced by the inclusion including Al2 O3 and TiN gradually increases with increasing the UTS, while the percentage of failure at sample surfaces decreases conversely and the fatigue strength first increases and then decreases. Besides, it is interestingly noted that the inclusion sizes at the cracking origin for TiN are smaller than that for Al2 O3 because the stress concentration factor for TiN is larger than that for Al2 O3 based on the finite element simulation. For the first time, a new fatigue cracking criterion including the isometric inclusion size line in the strength-toughness coordinate system with specific physical meaning was established to reveal the relationship among the UTS, fracture toughness, and the critical inclusion size considering different types of inclusions based on the fracture mechanics. And the critical inclusion size of Al2 O3 is about 1.33 times of TiN. The fatigue cracking criterion could be used to judge whether fatigue fracture occurred at inclusions or not and provides a theoretical basis for controlling the scale of different inclusion types for high-strength steels. Our work may offer a new perspective on the critical inclusion size in terms of the inclusion types, which is of scientific interest and has great merit to industrial metallurgical control for anti-fatigue design.
基金supported by the National Natural Science Foundations of China (No.50734008)the Fundamental Research Funds for the Central Universities (No.FRF-AS-11-003A)
文摘The change of inclusions and microstructure of 16Mn steel treated by Ce were observed,and the effect of austenitizing temperature on the microstructure was also examined.The results show that the inclusions are transformed from Si-Mn-Al composite oxide and MnS into AlCeO3,Ce2O2S,and MnS composite inclusions after being treated by Ce.Plenty of intragranular ferrites are formed in 16Mn steel containing～0.017wt% Ce.A large amount of intragranular acicular ferrites are formed after being austenitized for 20min at 1473 K.The prior austenite grain size fit for the formation of intragranular acicular ferrites is about 120μm.
基金Item Sponsored by National High Technology Research and Development Program(863 Program)of China(2006AA03Z514)Innovation Fund for Outstanding Scholar of Henan Province of China(0621000700)
文摘Speed increase and heavy haul of the railway freight car are the effective measures to raise the transport ability of railway, and they have been the new trend for railway freight car development. The increase of speed and axle load will lead to the increase of wheel rail contact stress, which will make a great change of stress distribution in the whole wheel rim. Goodier equation was employed to analyze the stress state around inclusion and cavity. In the operation of wheel-rail contact stress, stress concentration occurred in the pole of Al2O3 spherical inclusion, while it appeared on the equator of cavity. The critical inclusion sizes of 250 kN axle load freight car wheel at different velocities, a certain distance away from the tread, were calculated by Murakami equation. Comparing the inclusion in cast steel wheel with that in rolled steel wheel, it is found that the inclusion in cast steel wheel has much more excellent crack resistance than that in rolled steel wheel. In the meantime, in order to reduce the probability of rim crack, some suggestions were put forward.
基金supported by the National Natural Science Foundation of China(No.52104369)the China Postdoctoral Science Foundation(No.2021M700374).
文摘The realization of an ideal combination of mechanical and fatigue properties is prerequisites for practical application of titanium(Ti)microalloyed steel in automotive field.The fatigue behavior of four Ti microalloyed high-strength beam steels with different Ti contents was systematically studied.The results show that the content of microalloying element Ti has a significant effect on the fatigue properties,especially in the steel with a high Ti content.For the experimental Ti microalloyed steel,inclusion-induced crack initiation is the main fatigue failure mode.Different from general fatigue fracture mechanism in Ti-contained steel,no TiN,which is the most detrimental to fatigue behavior,was found in fatigue crack initiation area.However,the large-sized TiN and oxide complex inclusion with a core-shell structure is the dominant cause of fatigue fracture.Because of the intense-localized deformation at the interface between complex inclusion and matrix,the angular TiN in the outer shell has a serious deteriorating effect on the fatigue properties,which is consistent with the result of the Kernel average misorientation map.Besides,the modification effect of a small amount of MnS on large-sized inclusion is not obvious and has little effect on the fatigue behavior.For more practical guidance,the critical inclusion sizes of the experimental steels were also investigated by experimental extrapolation method.With the increasing tensile strength,the inclusion sensitivity of the experimental steels increases,leading to the small critical inclusion size.
