Effects of austempering temperature on microstructure and surface residual stress of carbidic austempered ductile iron(CADI) grinding balls

The effects of austempering temperature on microstructure and surface residual stress of carbidic austempered ductile iron(CADI) grinding balls were systematically investigated in this work. The microstructures were oberserved by optical metallography and analyized by X-ray diffraction. The surface residual stress measured by the cutting method is mainly composed of thermal stress and phase transformation stress. The thermal stress in grinding balls was determined by ANSYS simulation technique, and the surface phase transformation stress was obtained by subtracting the simulated surface thermal stress from the measured surface residual stress. Results show that all microstructures consist of ausferrite, white-bright zones(mixture of martensite and austenite), nodular graphite, and carbides. The distribution of ausferrite shows uniform. With the increase of austempering temperature, the volume fraction and carbon content of austenite increase, whereas the amount of white-bright zone decreases. In addition, the surface residual stress increases with the increase of austempering temperature. Only the tension exists at the austempering temperature of 200 ℃, and the pressure exists at the austempering temperature of 220-260 ℃. The thermal stress changes from the tension on the inside with the radius of 0-35 mm to the pressure on the outside with the radius of 35-62.5 mm, and the stress balance state presents at the radius of 35 mm. It is also found that the transformation stress is related to the content of carbon-rich austenite, and will reduce by 5.03 MPa accompanied with 1 vol.% increase of the austenite. The thermal compressive stress and the transformation tensile stress on the surface both decrease with the increase of the austempering temperature.

Influence of Hot Deformation and Subsequent Austempering on the Mechanical Properties of Hot Rolled Multiphase Steel

Influence of hot deformation and subsequent austempering on the mechanical properties of hot rolled multiphase steel was investigated. Thermo-mechanical control processing （TMCP） was conducted by using a laboratory hot rolling mill, where three different kinds of finishing rolling reduction, and austemperings with various isothermal holding duration were applied. The results have shown that a multiphase microstructure consisting of polygonal ferrite, granular bainite and larger amount of stabilized retained austenite can be obtained by controlled rolling processes. Mechanical properties increase with increasing the amount of deformation because of the stabilization of retained austenite. Ultimate tensile strength （σb）, total elongation （σ） and the product of ultimate tensile strength and total elongation （σb-σ） reach the maximum values （791 MPa, 36% and 28476 MPa%, respectively） at optimal processes.

Austempering of Hot Rolled Si-Mn TRIP Steels

The austempering after hot roiling in hot roiled Si-Mn TRIP （transformation-induced plasticity） steels was investigated. The mechanism of TRIP was discussed through examination of the microstructure and the mechanical properties of this kind of steel. The results showed that the strain induced transformation to martensite of retained austenite occurs in hot rolled Si-Mn TRIP steels. The sample exhibited a good combination of ultimate tensile strength and total elongation when it was held at the bainite transformation temperature after hot deformation. The stability of retained austenite increases with an increase in isothermal holding time, and a further increase in the hold- ing duration resulted in the decrease of stability. The mechanical properties were optimal when holding for 25 min, and tensile strength and total elongation reached the maximum values （774 MPa and 33 ;, respectively）.

Mechanical properties and wear resistance of ultrafine bainitic steel under low austempering temperature

The mechanical properties and wear resistance of the ultrafine bainitic steel austempered at various temperatures were investigated.Scanning electron microscopy(SEM)and X-ray diffraction were used to analyze the microstructure.The worn surfaces were observed via laser scanning confocal microscopy and SEM.Results indicated that,under low austempering temperatures,the mechanical properties differed,and the wear resistance remained basically unchanged.The tensile strength of the samples was above 1800 MPa,but only one sample austempered at 230°C had an elongation of more than 10%.The weight loss of samples was approximately linear with the cycles of wear and nonlinear with the loads.The samples showed little difference in wear resistance at different isothermal temperatures,whereas the thickness of their deformed layers varied greatly.The results are related to the initial hardness of the sample and the stability of the retained austenite.Meanwhile,the experimental results showed that the effect of austempering temperature on the wear resistance of ultrafine bainitic steel can be neglected under low applied loads and low austempering temperature.

EFFECT OF AUSTEMPERING ON TRANSFORMATION INDUCED PLASTICITY OF HOT ROLLED MULTIPHASE STEELS

Effect of austempering on the transformation induced plasticity （TRIP） of hot rolled multiphase steel was investigated. Polygonal ferrite, granular bainite, and a large amount of stabilized retained austenite could be obtained in the hot rolled multiphase steel. Strain induced martensite transformation （SIMT） of retained austenite and TRIP effectively occur under straining owing to austempering after hot rolling, and mechanical properties of the present steel remain at a relatively high constant value for austempering at 400℃. The mechanical properties of the steel exhibited a good combination of tensile strength （791MPa） and total elongation （36%） because the stability of retained austenite is optimal when the steel is held for 20min.

Effect of nodule count and austempering heat treatment on segregation behavior of alloying elements in ductile cast iron

The equilibrium partition ratio, k, has been measured for Mn, Mo, Si, Ni and Cu in a ductile iron with composition(wt.%): 3.45 C, 0.25 Mn, 0.25 Mo, 2.45 Si, 0.5Ni and 0.5Cu with different nodule counts obtained from different section sizes of13, 25, 75 mm in the as cast, austenitized(at 870 °C for times 1, 4 and 6 hours) and austempered(at 375 °C for times 1 to 1,440 min) samples. Results show that Mn and Mo segregate positively at cell boundaries, but Si, Ni and Cu concentrate in an inverse manner in the vicinity ofgraphite nodules and there is a depletion ofthese elements at cell boundaries. Segregation curves for Ni and Cu are more smooth than for Si. Carbide formation has been observed at cell boundaries. Based on the results, the partition ratios for all elements decrease with increasing the nodule count. More carbide with coarser morphology has been observed in the microstructure with a lower nodule count. Austenitization for a longer time can decrease partition ratio, but cannot eliminate it entirely. Increasing the austenitization temperature has the same effect. Austenitizing parameters have no significant effect on carbides volume fraction. The kinetics ofaustempering is faster in higher nodule counts and subsequently better mechanical properties including higher ductility, strength and toughness have been observed for all austempering conditions studied.

Effect of Cu content on microstructures and mechanical properties of ADI treated by twostep austempering process

The effect of Cu content on the microstructures and mechanical properties (yield strength, ultimate tensile strength, impact energy, fracture toughness) of austempering ductile iron (ADI) treated by two-step austempering process were investigated. High Cu content in nodular cast irons leads to a significant volume fraction of retained austenite in the iron after austempering treatment, but the carbon content of austenite decreases with the increasing of Cu content. Moreover, austenitic stability reaches its maximum when the Cu content is 1.4% and then drops rapidly with further increase of Cu. The ultimate tensile strength and yield strength of the ADI firstly increases and then decreases with increasing the Cu content. The elongation keeps constant at 6.5% as the Cu content increases from 0.2% to 1.4%, and then increases rapidly to 10.0% with further increase Cu content to 2.0%. Impact toughness is enhanced with Cu increasing at first, and reaches a maximum 122.9 J at 1.4% Cu, then decreases with the further increase of Cu. The fracture toughness of ADI shows a constant increase with the increase of Cu content. The influencing mechanism of Cu on austempered ductile iron (ADI) can be classified into two aspects. On the one hand, Cu dissolves into the matrix and functions as solid solution strengthening. On the other hand, Cu reduces solubility of C in austenite and contributes more stable retained austenite.

Effects of Holding Temperature for Austempering on Mechanical Properties of Si-Mn TRIP Steel

A new type of high strength steel containing a significant amount of stable retained austenite was obtained by austempering immediately after intercritical annealing.This sort of low carbon steel only contains alloying elements of silicon and manganese rather than nickel and chromium.Its mechanical properties were enhanced considerably due to strain-induced martensite transformation and transformation-induced plasticity(TRIP)of retained austenite when it was strained at temperatures between Msand Md,because retained austenite was moderately stabilized due to carbon enrichment by austempering.Austempering was carried out at different temperatures and 400 ℃ was found to be optimal.Tensile strength,total elongation and strength-ductility balance reached the maximum values and the product of tensile strength and total elongation exceeded 30 135 MPa % when the TRIP steel was held at 400 ℃ and strained at 350 ℃.

Effects of Austempering on the Mechanical Properties of the Hot Rolled Si-Mn TRIP Steels

The effect of austempering on the mechanical properties of the hot rolled Si- Mn TRIP steels was studied. The mechanism of transformation induced plasticity （TRIP） was discussed through the examination of the microstructure and the mechanical properties of the specimens. The results stow that the microstructures of the steels were comprised of polygonal ferrite, granular bainite and a significant amount of stable retained austenite. The specimen exhibits excellent mechanical properties for the TRIP effect. Isothermal holding time for austempering affects the stability of retained austenite. The mechanical properties such as tensile strength, total elongation and strength ductility balance reach their optimal values （ 776 MPa , 33% and 25608 MPa% , respectively） when the specimen is held at 400℃ for 25 min.

Effect of austempering time on microstructure and properties of a low-carbon bainite steel

The effect of austempering time after the bainitic transformation on the microstructure and property in a low-carbon bainite steel was investigated by metallography and dilatometry. The results showed that by prolonging the austempering time after the bainite transformation, the amount of large-size martensite/austenite islands decreased, but no significant change of the amount and morphology of bainite were observed. In addition, more austenite with a high carbon content was retained by prolonging the holding time at the bainite transformation temperature.Moreover, with a longer holding time, the elongation was improved at the expense of a small decrease in tensile strength. Finally, the Avrami equation B(RF) = 1-exp(-0.0499 × t^0.7616) for bainite reaction at 350℃ was obtained for the tested steel. The work provided a reference for tailoring the properties of low-carbon steels.

Microstructure and mechanical properties of twostep Cu-alloyed ADI treated by different second step austempering temperatures and times

Austempering ductile iron (ADI) is an attractive material due to its excellent comprehensive mechanical properties. However, the deficit in elongation and toughness always threatens its security application. Two-step austempering process is an effective way to improve elongation and toughness simultaneously. In the present work, the influence of the amount, morphology and distribution of ferrite and austenite on mechanical properties of ADI under different second-step austempering parameters has been analyzed. Results show that the amount of austenite and its carbon content decrease with increasing of second-step temperature. Carbide begins to precipitate as second-step austempering temperature reaches 380 °C. These factors together influence the mechanical properties of two-step Cu-alloyed ADI. Impact energy and fracture toughness are strongly affected by second-step austempering temperature, and are dramatically decreased with increase of second-step austempering temperature. Elongation remains constant when the second-step temperature is below 360 °C, and then it is rapidly decreased with further increase of second-step temperature. Strength is slightly influenced by second-step temperature. Ferrite morphology is not influenced by second-step austempering duration, while blocky retained austenite size is slightly decreased with the increasing of second-step austempering time. The amount of retained austenite is decreased while the carbon content of retained austenite is increased with the extending of second-step austempering time. The substructure of austenite is transformed from dislocation to twin when second-step austempering time exceeds 60 min. Strength and elongation are improved slightly with extending of second-step time. Impact energy and fracture toughness initially decrease with the extending of second-step time, and then remain constant when the time is longer than 60 min. This is a result of austenite content decreasing and carbon content of austenite increasing. The second-step austempering time mainly influences austenite content and its carbon content, which is a result of carbon diffusion behavior variation.

Austempering of hot rolled transformation-induced plasticity steels

Thermomechanical controlled processing （TMCP） was conducted by using a laboratory hot rolling mill. Austempering in the salt bath after hot rolling was investigated. The effect of isothermal holding time on mechanical properties was studied through examining of the microstructure and mechanical properties of the specimens. The mechanism of transformation-induced plasticity （TRIP） was discussed. The results show that the microstructure of these steels consists of polygonal ferrite, granular bainite, and a significant amount of stable retained austenite. Strain-induced transformation to martensite of retained austenite and TRIP occur in the hot rolled Si-Mn TRIP steels. Excellent mechanical properties were obtained for various durations at 400℃. Prolonged holding led to cementite precipitation, which destabilized the austenite. The mechanical properties were optimal when the specimen was held for 25 min, and the tensile strength, total elongation, and strength ductility balance reached the maximum values of 776 MPa, 33%, and 25608 MPa.%, respectively.

Effect of austempering parameters on microstructure and mechanical properties of horizontal continuous casting ductile iron dense bars

In the present research, the orthogonal experiment was carried out to investigate the influence of different austempering process parameters(i.e. austenitizing temperature and time, and austempering temperature and time) on microstructure and mechanical properties of LZQT500-7 ductile iron dense bars with 172 mm in diameter which were produced by horizontal continuous casting(HCC). The results show that the major factors influencing the hardness of austempered ductile iron(ADI) are austenitizing temperature and austempering temperature. The fraction of retained austenite increases as the austenitizing and austempering temperatures increase. When austenitizing temperature is low, acicular ferrite and retained austenite can be effi ciently obtained by appropriately extending the austenitizing time. The proper austmepering time could ensure enough stability of retained austenite and prevent high carbon austenite decomposition. The optimal mechanical properties of ADI can be achieved with the following process parameters: austenitizing temperature and time are 866 °C and 135 min, and austempering temperature and time are 279 °C and 135 min, respectively. The microstructure of ADI under the optimal austempering process consists of fi ne acicular ferrite and a small amount of retained austenite, and the hardness, tensile strength, yield strength, elongation and impact toughness of the bars are HBW 476, 1670 MPa, 1428 MPa, 2.93% and 25.7 J, respectively.

Chro-austempering treatment of a medium-carbon high-strength bainitic steel

The integrated processing of chromizing and austempering(termed chro-austempering)treatments was proposed.The microstructure and properties of a medium-carbon high-strength bainitic steel treated by chro-austempering treatments were investigated by metallography,scanning vibrating electrode technique,electrochemical workstation,and microhardness test.The results show that the high-strength bainitic steel with carbide-free bainite as matrix and the chromized layers on surfaces was successfully fabricated by chro-austempering treatment.The hardness of surface layers was about 3.5 times that of the bainite matrix.Meanwhile,the corrosion started from exposed bainitic matrix and proceeded along the depth direction,testifying that the surface corrosion resistance was significantly improved by chro-austempering treatment due to the formation of Cr_(7)C_(3)and(Cr,Fe)7_(7)C_(3)on the surface.

Effects of Austempering and Martempering Processes on Amount of Retained Austenite in Cr-Mo Steels (FMU-226) Used in Mill Liner

The presence of retained austenite gives rise to deterioration of the wear resistance and fracture strength of Cr-Mo steels in many cases. Thus, the effects of heat treatments including direct quenching, martempering, and austempering on the retained austenite existing in the microstrueture of these steels were investigated. Specimens were austenized at 950 ℃ followed by direct quenching using compressed and still air. The specimens were also isothermally quenched in salt bath at 200 and 300 ℃ for 2, 8, 30, and 120min. Microstructures of the specimens were studied using optical microscope （traditional black and white etching as well as color etching）, scanning electron microscope （SEM）, microhardness tester, and X-ray diffraction （XRD）. The results showed that the lowest amount of retained austenite in the microstructure was obtained in the specimens quenched isothermally at 300 ℃ for 120 min.

Influence of austempering process on microstructures and mechanical properties of V-containing alloyed ductile iron

The influence of austempering time and vanadium addition on microstructure and mechanical properties of the alloyed ductile iron has been investigated. The 0.30 wt% V-containing and V-free alloyed ductile irons were firstly austenitized at 850 ℃ for 1 h and then austempered in a salt bath at 300 ℃ for 2, 3 and 4 h, respectively. For the 0.3 wt% V-containing alloyed ductile iron, the transformation product （ausferrite） was finer, and a small amount of martensite and a large amount of stable austenite were obtained after austempering for 2 h, while higher hardness and compressive strength of 62.8 HRC and 3000 MPa were achieved. For the V-free alloyed ductile iron, lower hardness and compressive strength were measured to be 56.8 HRC and 2320 MPa. As the austempering time increases, the amount of stable austenite decreases in the V-containing ductile iron, typically for the start of the second stage formation （retained austenite （γτ） →α ＋ carbide）. Based on this, it is assumed that the optimal processing window （OPW） was narrowed due to the addition of 0.30 wt% V as compared to the V-free ductile iron. When the hardness of 0.30 wt% V-alloyed ductile iron was higher than 59 HRC, the highest wear resist- ance was obtained. The mechanical cutting plays a dominant role in abrasive wear process.

Enhanced strength and toughness in 40CrNiMo steels by austempering below martensite start temperature

The austempering above and below martensite transition temperature(M_(s))was employed in a medium-carbon low-alloy 40CrNiMo steel,and the bainite and martensite multiphase microstructures with different volume fractions were obtained.Here,the effect of pre-existing martensite on subsequent transformation of bainite microstructure and mechanical properties is focused and researched.The microstructure with a volume fraction of pre-existing martensite(V_(PM)),bainite(V_(B)),and martensite/austenite(V_(M/A))constituents of approximately 28%,46%,and 26%,respectively could be obtained by austenitizing below M_(s)(280℃)for 1 h,and an optimum combination of strength,ductility,and impact toughness(yield strength of 1420 MPa,ultimate tensile strength of 1795 MPa,total elongation of 7.9%,and V-notch impact value of 37 J)was achieved.The considerable enhancement of mechanical properties in the sample austenitized below M_(s)is mainly ascribed to the formation of the pre-existing martensite,resulting in an effective reduction in the size of the bainite plates and martensite/austenite constituents.

Microstructure and abrasive wear resistance of an alloyed ductile iron subjected to deep cryogenic and austempering treatments

To further improve the mechanical performance of a new alloyed austempered ductile iron（ADI）, deep cryogenic treatment（DCT） has been adopted to investigate the effect of DCT time on the microstructure and mechanical behaviors of the alloyed ADI Fe-3.55 C-1.97 Si-3.79 Ni-0.71 Cu-0.92 Mo-0.64 Cr-0.36 Mn-0.30 V（in wt.%）. With increasing the DCT time, more austenite transformed to martensite and very fine carbides precipitated in martensite in the extended period of DCT. The amount of austenite decreased in alloyed ductile irons, while that of martensite and carbide precipitation increased. The alloyed ADI after DCT for 6 h had the highest hardness and compressive strength, which can be attributed to the formation of more plate-like martensite and the finely precipitated carbides. There was a gradual decrease in hardness and compressive strength with increasing the DCT time to 12 h because of the dissolution of M3 C carbide. After tempering, there was a decrease in mechanical properties compared to the direct DCT sample, which was caused by the occurrence of Ostwald ripening of precipitated carbides. The optimum wear resistance was achieved for the alloyed ADI after DCT for 6 h. The wear mechanism of the alloyed ADI in associating with DCT is mainly consisted of micro-cutting wear and some plastic deformation wear.

Successive Boronizing and Austempering for GGG-40 Grade Ductile Iron

Boronizing and austempering were successively applied to a GGG-40 grade ductile iron in order to combine the advantages of both process in a single treatment. This new procedure formed a 30 μm thick boride layer on the surface with subsurface matrix structure consisted of acicular ferrite and retained austenite. Reciprocating wear tests showed that successive boronizing and austempering exhibited considerably higher wear resistance than conventional boronizing having a subsurface matrix structure consisting of ferrite and pearlite.

Precise control of bainite micro-morphology and the effects on the mechanical properties of ultrahigh-strength complex-phase sheet steel

Bainite, the main microstructure of ultrahigh-strength complex-phase(CP) sheet steel, usually exhibits various micro-morphologies when subjected to different austempering treatments.In the current study, conventional austempering treatment at the bainite nose temperature resulted in two bainite types with distinct micro-morphologies: polygonal blocky bainite and acicular bainite, which resulted in large fluctuations in the mechanical properties of CP steel, particularly yield strength and hole expansion ratio.Therefore, the precise control of bainite micro-morphology was studied and applied to separate the two bainite types through austempering optimization.The two bainite types of different micro-morphologies had different effects on the mechanical properties of CP steel: the acicular bainite favored hole expansion and flangeability but deteriorated ductility, while the polygonal blocky bainite favored ductility but deteriorated hole expansion and flangeability.Accordingly, two types of ultrahigh-strength CP steels of different mechanical properties can be stably manufactured through the precise control of bainitic micro-morphology to satisfy the specific demands of vehicle components in terms of the mechanical properties of CP steels.