A Study of Microstructure and Performance of Tempered Fe‐V‐W‐Mo Alloy

Influences of tempering temperature, holding time and tempering times on the microstructure and performance of Fe‐5%V‐5%W‐5%Mo‐5%Cr‐3%Nb‐2%Co(Fe‐V‐W‐Mo) were investigated by means of metallography, optical microscopy, hardness measurements, impact tester and pin abrasion tester. The results show that the hardness of Fe‐V‐W‐Mo alloy remains constant when tempered below 350°C. The hardness decreases gradually as the tempering temperature increase until around 475°C and then it increases again to a peak at 525°C. The hardness of Fe‐V‐W‐Mo alloy reaches nearly the highest value after the first tempering and decreases after triple‐tempering. The toughness of Fe‐V‐W‐Mo alloy increases until the tempering temperature reaches 475°C and then decreases until the temperature reaches 525°C. However, it increases again when tempering is beyond that temperature. The excellent wear resistance can be obtained by tempering at 500‐550°C.     

Development of High Strength Plates with Low Yield Ratio by the Combination of TMCP and Inter‐Critical Quenching and Tempering

A new processing route of thermo‐mechanical processing (TMCP) followed by inter‐critical quenching and tempering (L‐T) was developed to produce 590MPa grade high strength plates based on a relatively lean composition of plain carbon manganese steels microalloyed with Nb, V and Ti. The effect of quenching temperatures on the evolution of microstructure and mechanical properties were investigated. The nano‐hardness measurements of martensite were performed with a nano‐indenter, which indicated that the fractions of as quenched and tempered martensite increased and their hardness values decreased with increasing quenching temperatures in the range from 760 °C to 810 °C. For both as quenched and tempered samples, ferrite grain sizes decreased with increasing quenching temperature in almost linear relationships. The yield strength increased with increasing the fraction of martensite while the tensile strength remained almost unchanged, leading to the increase of yielding ratio with increasing quenching temperatures. The optimum quenching temperature was determined to be around 760 °C in terms of strengths and yield ratio.     

Iron‐Rich Iron‐Aluminium‐Molybdenum Alloys with Strengthening Intermetallic μ phase and R phase Precipitates

Single‐phase and two‐phase ternary Fe‐Al‐Mo alloys with Al contents of usually 10 ‐16 at.% and Mo contents up to 42 at.% have been studied with respect to hardness at room temperature, yield stress and fracture strain at room temperature and higher temperatures up to 1000 °C and oxidation at temperatures of 400 ‐ 1000 °C. Thse alloys are strengthened by precipitation of the metastable R phase and/or the stable m phase depending on composition and heat treatment; both are hard and brittle intermetallic phases. The yield stress as well as the brittle‐to‐ductile transition temperature increases with increasing Mo content to reach yield stresses above 1400 MPa with, however, fracture strains below 1 % at temperatures below 800 °C. The observed short‐term oxidation is similar to that of other Fe‐Al alloys.     

Microstructure and Properties of Hot Working Die Steel H13MOD

Compared with H13 steel, the influences of different heat treatment process on the microstructure and properties of the new type of hot working die steel H13MOD were studied. The results show that the complete austenitizing temperature of H13MOD is around 1030 °C and the quenching hardness achieves the maximum value at this temperature. While for H13, the complete austenitizing temperature is above 1100 °C and the quenching hardness rise constantly with the quenching temperature increasing. In quenching process, the undissolved MC carbides can prevent the coarsening of grain in both steels. With the rise of quenching temperature, when MC carbides dissolve completely, the grain grows quickly. The hardness and strength of H13MOD at higher tempering temperature (above 570 °C) are nearly the same as those of H13, but its toughness is higher than that of H13. Mo₂ C carbide is the main strengthening phase in H13MOD, which is attributed to the higher content of Mo. The quantity of VC eutectic carbides is reduced because of lower content of V in H13MOD, which plays an important role in enhancing the impact toughness of H13MOD. Under a certain strength condition, H13MOD steel can be used in the environment that higher toughness is required and the service life of die casting mold can be improved.     

Characteristics of Ti‐Ce Complex Deoxidation Products in a Fe‐20mass%Cr Alloy

The particle characteristics such as size distribution, composition and morphology have been studied in an Fe‐20mass%Cr alloy as a function of holding time at 1600°C. The alloy was deoxidised with Ti and Ce, followed by holding at 1600°C and cooling to 1400°C and quenching. The inclusion particles were investigated on a surface of film filter with an open pore size of 0.05 or 5 μm after electrolytic extraction of the metal samples. Different electric charge and electrolytes (2%TEA and 10%AA) were compared for extraction of the Fe‐20mass%Cr alloy. 300 Coulombs with 10%AA was found most suitable for the electrolytic extraction of particles to determine the particle composition and size distribution. Most of the particles were found to be complex oxides containing Ti, Ce and Cr. Furthermore, the composition of the particles was found to change from a high Ce‐content to a high Cr‐content with longer holding time at 1600°C.     

Toughness variation with test temperature and

A two phase heavy alloy composite based on 95 W-3.5 Ni-1.5 Fe (wt pct) was fabricated from elemental powders by liquid phase sintering. Past reports on the heavy alloys indicate considerable disagreement concerning cooling rate effects on toughness. The present experiments determined the effect of both cooling rate and test temperature on the properties of the 95 W heavy alloy. This alloy undergoes a ductile to brittle transition with decreasing test temperature; the transition temperature is close to room temperature. The cooling rate from post-sintering anneals carried out at temperatures greater than 1000 °C has a large influence on toughness; rapid quenching gives superior toughness. These findings support an impurity segregation explanation for embrittlement in the heavy alloys.     

Correlation between Microstructure and Mechanical Properties of High C‐Cr Cast Steel

The effect of carbide morphology and matrix structure on abrasion resistance of cast alloyed steel with 2.57% C, 16.2% Cr and 0.78% Mo was studied in the as‐cast and heat treated conditions. Samples were austenitized at three different temperatures of 980, 1050 and 1250 °C for 15 minutes and followed by tempering at 540 °C for 3 hours. The austenitizing temperature of 980 °C revealed fully martensitic structure with little amount of retained austenite, while at 1050 °C the matrix was austenitic with massive amount of coarse secondary carbides. The austenitic matrix with very fine secondary carbides was developed at 1250 °C. The maximum abrasion resistance was obtained at 1050 °C due to the highest structure hardness and existence of both eutectic and secondary carbides in larger size than the formed groove by the abrasive particles during the wear test. On the other hand, the as‐cast pearlitic structure showed high wear rate by an applied load of up to 0.2 bar, followed by very rapid increase in wear rate with higher applied loads. It could be considered that the austenitizing temperature of 1050 °C showed better combination of abrasion resistance and toughness in comparison with other heat treatment cycles.     

Design of strong tough Fe/Mo/C martensitic steels and the effects of cobalt

The microstructures and mechanical properties of a series of vacuum melted Fe/(2 to 4) Mo/(0.2 to 0.4) C steels with and without cobalt have been investigated in the as-quenched fully martensitic condition and after quenching and tempering for 1 h at 673 K (400°C) and 873 K (600°C); austenitizing was done at 1473 K (1200°C) in argon. Very good strength and toughness properties were obtained with the Fe/2 Mo/0.4 C alloy in the as-quenched martensitic condition and this is attributed mainly to the absence of internal twinning. The slightly inferior toughness properties compared to Fe/Cr/C steels is attributed to the absence of interlath retained austenite. The two 0.4 pct carbon steels having low Mo contents had approximately one-half the amount of transformation twinning associated with the two 0.4 pct carbon steels having high Mo contents. The plane strain fracture toughness of the steels with less twinning was markedly superior to the toughness of those steels with similar alloy chemistry which had more heavily twinned microstructures. Experiments showed that additions of Co to a given Fe/Mo/C steel raised M_s but did not decrease twinning nor improve toughness. Molybdenum carbide particles were found in all specimens tempered at 673 K (400°C). The Fe/Mo/C system exhibits secondary hardening after tempering at 873 K (600°C). The precipitate is probably Mo₂C. This secondary hardening is associated with a reduction in toughness. Additions of Co to Fe/Mo/C steels inhibited or eliminated the secondary hardening effect normally observed. Toughness, however, did not improve and in fact decreased with Co additions.     

Design of strong tough Fe/Mo/C martensitic steels and the effects of cobalt

The microstructures and mechanical properties of a series of vacuum melted Fe/(2 to 4) Mo/(0.2 to 0.4) C steels with and without cobalt have been investigated in the as-quenched fully martensitic condition and after quenching and tempering for 1 h at 673 K (400°C) and 873 K (600°C); austenitizing was done at 1473 K (1200°C) in argon. Very good strength and toughness properties were obtained with the Fe/2 Mo/0.4 C alloy in the as-quenched martensitic condition and this is attributed mainly to the absence of internal twinning. The slightly inferior toughness properties compared to Fe/Cr/C steels is attributed to the absence of interlath retained austenite. The two 0.4 pct carbon steels having low Mo contents had approximately one-half the amount of transformation twinning associated with the two 0.4 pct carbon steels having high Mo contents. The plane strain fracture toughness of the steels with less twinning was markedly superior to the toughness of those steels with similar alloy chemistry which had more heavily twinned microstructures. Experiments showed that additions of Co to a given Fe/Mo/C steel raisedM _S but did not decrease twinning nor improve toughness. Molybdenum carbide particles were found in all specimens tempered at 673 K (400°C). The Fe/Mo/C system exhibits secondary hardening after tempering at 873 K (600°C). The precipitate is probably Mo₂C. This secondary hardening is associated with a reduction in toughness. Additions of Co to Fe/Mo/C steels inhibited or eliminated the secondary hardening effect normally observed. Toughness, however, did not improve and in fact decreased with Co additions.     

Study of New On‐Line Pre‐Hardening Technology of 3Cr2NiMo Plastic Die Steel

The thermal property parameters of 3Cr2NiMo plastic die steel were tested and the two‐dimensional finite element model was established in the paper. The temperature fields of 3Cr2NiMo plastic die steel plate with the thickness of 130 mm were calculated and analyzed under three quenching processes. The results showed that for all of the three processes the cooling rates could avoid the pearlite transformation zone when the temperature of the steel plate was more than 500°C. When the temperature was less than 500°C, the cooling rate of the third process was slower, and the temperature difference was effectively alleviated, which avoided large transformation stress appearance. For 3Cr2NiMo plastic die steel with the thickness of 130 mm, the third process was the best process. In the paper, the hardness and the microstructures were tested after the third process and tempering. After tempering process, the structures were all tempered sorbite, and the tempering hardness difference in the whole steel plate was less than 3HRC, and there were no cracks. The study provided references for on‐line pre‐hardening process formulation and optimization of large plastic die steel.     

Iron‐Rich Iron‐Titanium‐Silicon Alloys with Strengthening Intermetallic Laves Phase Precipitates

Two‐phase ternary Fe‐Ti‐Si alloys with Si contents from 2 to 16 at.% and Ti contents from 2 to 28 at.% were studied with respect to room temperature hardness, fracture strain and yield stress at room and higher temperatures up to 1150 °C. In addition oxidation was checked at temperatures between 400 and 1150 °C. The alloys are strengthened by precipitation of the stable Laves phase (Fe,Si)₂Ti which is a hard and brittle intermetallic phase. The yield stress as well as the brittle‐to‐ductile transition temperature (BDTT) increase with increasing Ti content. Yield stresses up to about 1400 MPa and BDTT between 100 °C and 600 °C with fracture strains of the order of 1 % below BDTT were achieved. The observed short‐term oxidation performance at temperatures up to 1150 °C compares favourably with that of Fe‐AI alloys with high Al contents.     

On Phase Equilibria in Duplex Stainless Steels

The equilibrium conditions of four duplex stainless steels; Fe‐23Cr‐4.5Ni‐0.1N, Fe‐22Cr‐5.5Ni‐3Mo‐0.17N, Fe‐25Cr‐7Ni‐4Mo‐0.27N and Fe‐25Cr‐7Ni‐4Mo‐1W‐1.5Cu‐0.27N were studied in the temperature region from 700 to 1000 °C. Phase compositions were determined with SEM EDS and the phase fractions using image analysis on backscattered SEM images. The results showed that below 1000 °C the steels develop an inverse duplex structure with austenite and sigma phase, of which the former is the matrix phase. With decreasing temperature, the microstructure will be more and more complex and finely dispersed. The ferrite is, for the higher alloyed steels, only stable above 1000 °C and at lower temperatures disappears in favour of intermetallic phases. The major intermetallic phase is sigma phase with small amounts of chi phase, the latter primarily in high Mo and W grades. Nitrides, not a focus in this investigation, were present as rounded particles and acicular precipitates at lower temperatures. The results were compared to theoretical predictions using the TCFE5 and TCFE6 databases.     

Deformation Behaviour of Iron‐Rich Iron‐Aluminium Alloys with Ternary Transition Metal Additions

The hardness and yield stress at room temperature and the brittle‐to‐ductile transition temperature of Fe‐Al alloys with 16 at.% Al, which is in the range of the so‐called K‐state with possible short‐range ordering reactions, and ternary additions of 0.5 and 4 at.% of the transition metals Cr, Mo, Mn, V, Ti and Ni were studied with respect to possible hardening effects of the ternary additions. The addition of Cr, Mo and Mn to the Fe‐Al alloys produce solid‐solution hardening which corresponds to the hardening effect of Al. Only Ti, V and Ni produce extra hardening effects which cannot be related to solid‐solution hardening. This extra hardening is attributed to possible fine NiAl precipitates in the Fe‐Al‐Ni case and to possible enhanced short‐range ordering and/or fine carbide precipitates in the cases of Fe‐Al‐V and Fe‐Al‐Ti.     

Mechanical properties of new low‐nickel cobalt‐free maraging steels

To study the mechanical properties of newly developed low‐nickel cobalt‐free maraging steels, six laboratory series with varied Ti, Mo, and Cr mass contents (0.003‐ 1.65, 0.0074‐ 5 and 0.004‐ 5 %) were studied. The study showed that increasing Ti and Mo contents improve the tensile strength and yield strength without affecting ductility markedly. The chromium alloyed grades M2, M6, M8 and M12 exhibit properties comparable to those of T 250 grade. To investigate the effects of heat treatment temperature on the mechanical properties, the solution treatment was carried out at temperatures varying between 820 and 1100 °C and holding times of 15 min at different cooling rates. The ageing temperature varied from 400 to 500 °C at ageing times of 60, 120 and 240 min, respectively. The ageing response of hardness as a function of temperature at different soaking conditions was determined to optimise the heat treatment conditions. The mechanical properties of the different steels after solution treatment and ageing at optimum temperature and time were determined.     

Galvannealing of (High‐)Manganese‐Alloyed TRIP‐ and X‐IP®‐Steel

In this study the influence of Mn on galvannealed coatings of 1.7% Mn‐1.5% Al TRIP‐ and 23% Mn X‐IP®‐steels was investigated. It is shown that the external selective oxides like Mn, Al and Si of the TRIP steel which occur after annealing at 800 °C for 60 s at a dew point (DP) of ‐25 °C (5% H₂) hamper the Fe/Zn‐reaction during subsequent galvannealing. Preoxidation was beneficially utilized to increase the surface‐reactivity of the TRIP steel under the same dew point conditions. The influence of Mn on the steel alloy was investigated by using a 23% Mn containing X‐IP®‐steel which was bright annealed at 1100 °C for 60 s at DP ‐50 °C (5% H₂) to obtain a mainly oxide free surface prior to hot dip galvanizing (hdg) and subsequent galvannealing. As well known from the literature Mn alloyed to the liquid zinc melt stabilizes δ‐phase at lower temperatures by participating in the Fe‐Zn‐phase reactions, it was expected that the metallic Mn of the X‐IP®‐steel increases the Fe/Zn‐reactivity in the same manner. The approximation of the effective diffusion coefficient (D_eff(Fe)) during galvannealing was found to be higher than compared to a low alloyed steel reference. Contrary to the expectation no increased Fe/Zn‐reaction was found by microscopic investigations. Residual η‐ and ζ‐phase fractions prove a hampered Fe/Zn‐reaction. As explanation for the observed hampered Fe/Zn‐reaction the lower Fe‐content of the high‐Mn‐alloyed X‐IP®‐steel was suggested as the dominating factor for galvannealing.     

Effect of molybdenum on the fracture characteristic of high-speed steel quenched matrix

The fractures of three model alloys, imitating by their chemical composition the matrixes of the quenched high-speed steels of various Mo: W relations were analyzed. According to the measurements of the stress intensity factor K_Ic and the differences in the precipitation processes of carbides it was found out that the higher fracture toughness of the matrix of the molybdenum high-speed steels than on the tungsten ones is the results of the differences in the kinetics of precipitation from the martensite matrix of these steels during tempering. After tempering at 250 and 650°C the percentage of the intergranular fracture increases with the increase of the relation of Mo to W in the model alloys of the high-speed steel matrix. This is probably the result of higher precipitation rate of the M₃C carbide (at 250°C) and the MC and M₆C carbides (at 650°C) in the privileged regions along the grain boundaries. The change of the character of the model alloy fractures after tempering at 450°C from the completely transgranular one in the tungsten alloy to the nearly completely intergranular one in the molybdenum alloy indicates that the coherent precipitation processes responsible for the secondary hardness effect in the tungsten matrix begin at a lower temperature than in the molybdenum matrix. After tempering for the maximum secondary hardness the matrix fractures of the high-speed steels reveal a transgranular character regardless the relation of Mo to W. The higher fracture toughness of the Mo matrix can be the result of the start of the coherent precipitation processes at a higher temperature and their intensity which can, respectively, influence the size of these precipitations, their shape and the degree of dispersion. The transgranular character of the fractures of the S 6-5-2 type high-speed steel in the whole range tempering temperatures results from the presence of the undissolved carbides which while cracking in the region of stress concentration can constitute flaws of critical size which form the path of easy cracking through the grains. The transgranular cracking of the matrix of the real high-speed steels does not change the adventageous influence of molybdenum upon their fracture toughness. On the other hand, the carbides, undissolved during austenitizing, whose size distribution in the molybdenum steels from the point of view of cracking mechanics seems to be unsatisfactory, influence significantly the fracture toughness of these steels.     

Intragranular Ferrite Formation Mechanism and Mechanical Properties of Non‐quenched‐and‐tempered Medium Carbon Steels

The type and size distribution of inclusions in non quenched‐and‐tempered medium carbon steels were investigated quantitatively. The effect of Ti containing complex inclusions on the formation of intragranular ferrite was studied. The continuous cooling transformation (CCT) diagrams of the tested steels were obtained for determining the cooling rate range of the formation of intragranular ferrite. The mechanical properties of the tested steels were determined at room temperature. The results show that with increasing Ti content, the fraction of Ti containing complex inclusions, which could act as nuclei for intragranular ferrite, increased and the inclusion size became smaller. In the cooling rate range of 0.5~2.5°C/s, plenty of intragranular ferrite formed, while at the rate of 2°C/s, the microstructure was mainly acicular ferrite. With the formation of intragranular ferrite, the toughness was enhanced by about 50% keeping the same strength level for the studied steels.     

An Assessment of Vacuum‐Heat‐Treated H11 Hot‐Work Tool Steel using the KIc/HRc Ratio

Charpy V‐notch (CVN) impact‐test values are widely used in toughness specifications for AISI H11 hot‐work tool steel, even though the fracturing energy is not directly related to the tool design. K_Ic, the plain‐strain stress‐intensity factor at the onset of unstable crack growth, can be related to the tool design; however, K_Ic test values are not widely used in toughness specifications. This is surprising since to the designer K_Ic values are more useful than CVN values because the design calculations for tools and dies of high‐strength steels should take into account the strength and the toughness of materials in order to prevent the possibility of rapid and brittle fracture. An investigation was conducted to determine whether standardized fracture‐toughness testing (ASTM E399‐90), which is difficult to perform reliably for hard materials with a low ductility, could be replaced with a so far non‐standard testing method. A particular problem is that the manufacture of the fatigue crack samples is difficult and expensive, and this has promoted the search for alternative fracture‐toughness testing methods. One of the most promising methods is the use of circumferentially notched and fatigue‐precracked tensile specimens. With this technique the fatigue crack in the specimen is obtained without affecting the fracture toughness of the steel, if it is obtained in soft annealed steel, i.e., prior to the final heat treatment. The results of this investigation have shown that using the proposed method it was possible to draw, for the normally used range of working hardness, combined tempering diagrams (Rockwell‐C hardness ‐ Fracture toughness K_Ic ‐ Tempering temperature) for some AISI H11 hot‐work tool steel delivered from three steel plants. On the basis of the combined tempering charts the influence of the processing route on the mechanical properties was investigated. In the same way, vacuum‐heat‐treated tool steels were assessed and their properties expressed as a ratio of the fracture toughness to the hardness (K_Ic/HRc).     

Strengthening and toughening treatment of steel used as the material of multistation progressive dies

The Cr12MoV steel was chosen as the material of multistation progressive dies, and its impact toughness and hardness were measured, following different heat treatments. Based on experimental results, a practical technology carried out in vacuum is recommended as the strengthening and toughening treatment for these dies: 980°C oil pre-quenching, oil quenching again at 980°C, final tempering at 200°C for 1 hour, and then air cooling. This treatment makes the steel have fine austenitic grains and carbide particles, and thus leads to good combination of strength and toughness such that the service life of the dies is increased.     

Optimizing structure,toughness and wear performance of 15 % Cr cold work cast tool steel

A series of six Cr-, Cr + Mo-, Cr + Mo + V cold work cast tool steels were produced and investigated for microstructure, impact toughness and both experimental and industrial abrasive wear. Grain refinement of the steel matrix even in as-cast condition was obtained on using 2.3 % Mo + 0.9 % V and that ensured increasing impact toughness and abrasion resistance. An optimum impact toughness of about 85 J-cm^?2 was obtained in air quenched (970°C) and tempered (450°C) Mo + V containing steels in which area fraction of carbides reached 38 %. The abrasion resistance improved in case of steels tempered at 250°C and had fine grain structure.