The effects of diferent heat and cryogenic (sub-zero) treatment parameters such as temperature and holding time on the microstructure (amount of retained austenite) and hardness of extrusion molds produced from the 21NiCrMo2 and 100Cr6 steels were investigated. The 21NiCrMo2 grade extrusion die was carburized for 22.5 h in an endogas (25% CO, 35% N2, 40% H2) atmosphere at 920 °C. At the end of the carburization process, the temperature was kept at 850 °C, which is the austenitization temperature, for 2 h, followed by cooling in oil at 80 °C and remaining in oil for 45 minutes. The carburizing process was not performed for the extrusion molds made of 100Cr6 steel grade. Only the austenitizing heat treatment at 850 °C (holding for 2 h) was carried out in this steel. The steel molds which were produced with 21NiCrMo2 and 100Cr6 steels were cryogenically treated at -120 °C for 2 h and subsequently tempered at 150 °C for 1.5 h. As a result of the cryogenic treatment, the hardness of 21NiCrMo2 steel increased to 840 Hv and the wear resistance of the extrusion die surface was improved. The amount of residual austenite decreased from 20% to 6% after the cryogenic treatment. Due to the effect of the cryogenic process, the surface hardness of the 100Cr6 steel sample increased to ~870 Hv, which implies an increase of 4.5%, due to the transformation of residual austenite to martensite. The mass loss, during the wear tests, of the hardened extrusion dies was reduced from 0.1420 mg to 0.0221 mg. The notch impact strength value measured in this condition was 20 J. The 100Cr6 steel after the cryogenic treatment was used to extrude 12 tons of Al alloy in an industrial press. This amount of material is 30% lower than for hot work tool steel. On the other hand, the 100Cr6 steel is more economical and heat treatment is more practical. The extrusion performance of 21NiCrMo2 steel was 50% lower than the hot work tool steel.
Se ha investigado los efectos de diferentes parámetros de tratamiento térmico y criogénico como la temperatura y el tiempo de mantenimiento sobre la microestructura (cantidad de austenita retenida) y la dureza de moldes de extrusión producidos a partir de los aceros 21NiCrMo2 y 100Cr6. La matriz de extrusión del acero 21NiCrMo2 se cementó durante 22,5 h en una atmósfera de gas (25% CO, 35% N2, 40% H2) a 920 °C. Al final del proceso de cementación, la temperatura se mantuvo a 850 °C, que es la temperatura de austenización, durante 2 h, seguido de enfriamiento en aceite a 80 °C, permaneciendo en aceite durante 45 minutos. No se realizó este proceso de cementación para los moldes de extrusión fabricados con el acero 100Cr6. En este acero solo se llevó a cabo el tratamiento de austenización a la temperatura de 850 °C (manteniendo durante 2 h). Los moldes de acero que se fabricaron con aceros los 21NiCrMo2 y 100Cr6 se trataron posteriormente de manera criogénica a -120 °C durante 2 h y, posteriormente, se templaron a 150 °C durante 1,5 h. Como resultado del tratamiento criogénico, la dureza del acero 21NiCrMo2 aumentó hasta los 840 Hv y mejoró la resistencia al desgaste de la superficie de la matriz de extrusión. La cantidad de austenita residual disminuyó del 20% al 6% después del tratamiento criogénico. Por efecto del proceso criogénico, la dureza superficial de la muestra de acero 100Cr6 aumentó a ~870 Hv, lo que supone un incremento del 4,5%, debido a la transformación de la austenita residual a martensita. La pérdida de masa durante el ensayo de desgaste de las matrices de extrusión endurecidas se redujo de 0,1420 mg a 0,0221 mg. El valor de resistencia al impacto medido en esta condición fue de 20 J. El acero 100Cr6 después del tratamiento criogénico se usó para extruir 12 toneladas de aleación de Al en una prensa industrial. Esta cantidad de material es un 30% inferior a la del acero para herramientas para trabajo en caliente. Por otro lado, el acero 100Cr6 es más económico y el tratamiento térmico es más práctico. El rendimiento durante el proceso de extrusión del acero 21NiCrMo2 fue un 50% inferior al del acero para herramientas de trabajo en caliente.
The application of cryogenic treatments to metals has recently been recognized as an effective method to increase the “wear resistance” and reduce residual stresses in tool or die steels (
The process flow diagram is organized in the form of turning, milling and grinding, respectively. The steel was first turned into a lathe by cutting the front/back sides of the mill and the milling process was completed with the threading process. Finally, the production of the mold is completed by grinding of the outer corner and the grinding of the bearings. The chemical composition of the steels used to produce the extrusion molds is shown in
C | Mn | Ni | Cr | Mo | Si | S | P | Cu | |
---|---|---|---|---|---|---|---|---|---|
|
0.21 | 0.77 | 0.43 | 0.55 | 0.18 | 0.20 | 0.02 | 0.026 | - |
|
0.91 | 0.33 | - | 0.47 | 0.06 | 0.27 | 0.01 | 0.023 | 0.29 |
Extrusion mold produced using the two steels under investigation were grinded and different heat treatment cycles have been applied. 21NiCrMo2 samples grade extrusion molds were carburized at 920 °C in endogas (25% CO, 35% N2, 40% H2) atmosphere for 22.5 h. The extrusion molds made of 100Cr6 was not subjected to the carburizing because it has enough carbon content. 100Cr6 extrusion molds were heated to the austenitizing temperature of 850 °C and held for 2 h. Then all samples (21NiCrMo2, 100Cr6 extrusion molds) were hardened in oil at 80 °C by promoting the formation of martensite after cooling to this temperature (and held for 45 min). After the hardening process, samples were cryogenically treated by using liquid Nitrogen at -120 °C for 2 h. Then samples were tempered at 150 °C for 2.5 h to minimize the residual stresses in the extrusion mold. The shallow cryogenic treatment which undertaken between -80 °C and -130 °C was performed. The shallow cryogenic treatment is defined as the most proper process for this alloy.
Material | Carburized | Austenitized | Oil Quench | Cryogenic | Tempered |
---|---|---|---|---|---|
|
+ | + | + | + | + |
|
- | + | + | + | + |
All extrusion die steels, which were subjected to cryogenic treatments and then tempered, were made ready for microstructure examination by standard metallographic methods and then the samples were etched using 2% Nital solution. Afterwards, examinations were carried out with optical and scanning electron microscopes. The Tescan Vega test device was used.
The hardness values were measured on the cross-sectional area from the extrusion molds. Micro hardness test were performed using a 1 kgf load. Hardness measurements were tested from the surface to the center of the sample. Hardness measurement values are obtained separately for each heat treatment.
For the wear tests, wear test samples were undertaken on the cross-sectional area on the extrusion mold under untreated and heat treated conditions. Pin-on disc type of test apparatus (Koehler Instrument, K93590 Pin on Disc Tester 230V, 50 Hz) was used for the wear tests. These tests were conducted in accordance with the
Notch impact test were conducted on a Izod Charpy test device with a capacity of 350 J under different conditions. Test specimens were adjusted to the dimensions: 55×10×10 mm3 as a standard. The experiments performed on the treated samples were conducted at an ambient temperature of 18 °C and in 45% humid air atmosphere and each test was repeated twice. As a result of the impact tests, the refractive surface morphology was examined by performing SEM analyses.
The performance of the extrusion molds afer applying the heat and cryogenic treatments under factory operating conditions was examined. For these experiments, extrusion die life and billet production capacities obtained under routine production conditions were compared with cryogenic process die extrusion performances. Routine casting and extrusion parameters were used in the production of 6000 series Al alloys in the company. First of all, the homogenisation process of the 6060 aluminium billets was carried out and then the extrusion process was performed for the aluminium profile production. The mold without heat treatment was connected to the extruder and extruded with an extrusion speed of 5 mm·s-1 by applying 30 and 75 extrusion rates (R). The extrusion speed was controlled with the aid of the punch advance-time curve. The billet and shell temperature, which are defined as process parameters, were kept constant at 450 °C and 400 °C respectively.
(Symbol RA stands for retained austenite)
The microstructure of these alloys after the heat and cryogenic treatments contained retained austenite and martensite phase which is a metastable low temperature phase (
The plots in
a) 21NiCrMo2; b) 100Cr6
The friction coefficient-wear distance graphs of the untreated and hardened samples against Al2O3 balls are given in
Untreated | Hardened | Cryogenic | Tempered | ||
---|---|---|---|---|---|
Wear Properties | 500 m | 500 m | 500 m | 500 m | |
21NiCrMo2 | Loss of Sample (mg) | 0.1521 | 0.0329 | 0.0178 | 0.0255 |
Friction Coefficient | 0.6321 | 0.4509 | 0.2865 | 0.3209 | |
100Cr6 | Loss of Sample (mg) | 0.1420 | 0.0294 | 0.0167 | 0.0221 |
Friction Coefficient | 0.5864 | 0.3661 | 0.2367 | 0.2789 |
Distance: 500 m; a) untreated, b) hardened, c) sub-zero, d) tempered samples
The Impact test results of steels 21NiCrMo2 and 100Cr6 extrusion molds after the different heat and cryogenic treatments is given in
The untreated steel samples had a highest impact energy. The hardness increased and the toughness (absorbed impact energy) decreased with the application of the different treatments. The notch impact strength values vary in the range from 250 J to 20 J. SEM analyzes of the fracture surfaces were performed and are shown in
a) untreated, b) hardened, c) sub-zero, d) tempered (scale bar is 50 µm)
a) untreated, b) hardened, c) sub-zero, d) tempered (scale bar is 50 µm)
a) isometric view, b) front surface of die and shape of profile
The extrusion molds used in the experiments are shown in
From wear optical microscopy images (
Although it is possible that the performance of steels with a cryogenic treatment may decrease over time at production capacities of 18 tons, it has been concluded that it may be effective in the production of low tonnage profiles in the short term.
The deep cryogenic treatment (-120 °C) of quenched and tempered 21NiCrMo2, 100Cr6 steels improves the hardness. The amount of retained austenite is reduced by 75% after the cryogenic process performed after the tempered process for each steel. It is observed that the increase in the amount of residual austenite adversely affects the hardness. The maximum reduction in the amount of residual austenite was observed in the 100Cr6 steel. In the carburized 21NiCrMo2 die steel, residual austenite remained due to the high carbon content in the surface, which has been observed to reduce the wear resistance and fatigue limit of the extrusion dies. As a result, the carbon content of the molds is below 0.7 wt.% C and the quenching temperature (Tq) is lower than the martensite end temperature (Mf), providing a high amount of phase transformation. The steel extrusion dies were subjected to a wear test at a distance of 500 m under a load of 30 N, and the cryogenic hardening process was found to increase the wear resistance. The lowest weight loss (best wear resistance) was 0.0221 mg at a distance of 500 m in the 100Cr6 steel. The notch impact strength values have been brought from the level of 250 J to 20 J. Also SEM analyzes of the fracture surfaces after the impact tests were performed for 21NiCrMo2 and 100Cr6.
As an alternative to existing extrusion dies, it was shown that 100Cr6 steel, provided AA6000 series aluminum extrusion with a capacity of 12 tons. This value is 20-33% lower than the hot work tool steels mold performance. However, it has been concluded that it can be used to extruded some aluminium products which do not request sensitive surface properties.