| 1. |
- Rehan, Arbab, et al.
(författare)
-
Effect of Austenitization and Tempering on the Microstructure and Mechanical Properties of a 5 wt% Cr Cold Work Tool Steel
- 2016
-
Ingår i: Steel Research International. - : Wiley. - 1611-3683 .- 1869-344X. ; 12:1 December, s. 1609-1618
-
Tidskriftsartikel (refereegranskat)abstract
- The effects of austenitization and tempering temperatures for a 5 wt% Cr cold work tool steel are studied with an aim of understanding the influence on microstructure and mechanical properties. Microstructures are characterized with scanning electron microscopy and light optical microscopy. Retained austenite contents and martensite start temperatures are measured by X-ray diffraction and dilatometry, respectively. Hardness, impact toughness, and compressive yield strength are also determined. When the austenitization temperature is increased from 1020 or 1050 to 1075 °C, followed by tempering at 525 °C, significant hardness is gained while there is no increase in compressive yield strength. Higher austenitization temperatures also produce larger amounts of retained austenite. At the same time, the impact toughness is reduced due to coarsening of the martensitic microstructure. When the steel is tempered at 200 °C, a higher impact toughness and a higher volume fraction of retained austenite are observed. Retained austenite is not found after tempering at temperatures of 525 °C or above. It is concluded that the best combination of mechanical properties is achieved by austenitization at 1020 or 1050 °C followed by tempering at 525 °C.
|
|
| 2. |
- Rehan, Arbab
(författare)
-
Effect of heat treatment on microstructure and mechanical properties of a 5 wt.% Cr cold work tool steel
- 2019
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This work presents investigations related to phase transformations occurring inthe 5 wt.% Cr cold work tool steel Caldie during hardening and tempering treatments. The influence of austenitisation temperature, cooling rate, sub-zero cooling, isothermal treatment during cooling, tempering temperature and holding time on the microstructure and mechanical properties were investigated.The hardened microstructure of the investigated steel consisted of a mixture ofplate and lath martensite, minor amounts of bainite, blocky and thin retained austenite and M7C3 carbides. Increasing austenitisation temperature from 1020°Cto 1050°C was found useful as it provided higher hardness, good compressive strength and sufficient toughness. However, a further increase to 1075°C resulted in large prior austenite grains which produced coarse martensite containing somewhat increased carbon content. This was found to reduce the impacttoughness of the steel. Significant amounts of retained austenite were present after tempering for 2x2 h between 200°C and 500°C while tempering at 525°C or higher, reduced retained austenite content to below 2%. During holding at tempering temperature carbides precipitated in martensite and possibly in retained austenite. The retained austenite was thereby destabilised and transformed to martensite on cooling. This fresh martensite was tempered by following tempering treatments. It was concluded that tempering at 525°C for 2x2 h was suitable to achieve a good combination of hardness, compressive strength and impact toughness. Retained austenite was also found to transform during holding at 600°C for longer times. Initially, carbides formed in the austenite and after some time transformation of retained austenite to ferrite and carbides took place. Results were used to discuss alternative heat treatment procedures for the 5wt.% Cr cold work tool steel Caldie and some changes of current heat treatment recommendations were suggested.
|
|
| 3. |
- Rehan, Arbab, et al.
(författare)
-
Effect of microstructure on the impact toughness of a cold work tool steel
- 2014
-
Ingår i: Sonderbände der Praktischen Metallographie. - : Riederer. - 9783883554037 ; , s. 307-312
-
Konferensbidrag (refereegranskat)abstract
- The effect of different combination of austenitization temperatures and tempering treatments on microstructure and impact toughness has been studied for the cold work tool steel -Uddeholm Caldie. This is a chromium, vanadium and molybdenum alloyed tool steel with approximately 0.7 wt.% C. It is used as a tool in industrial applications such as cutting, punching, shearing, forming, drawing and extrusion. A combination of high hardness and adequate toughness is required to withstand the common tool failure modesof chipping and cracking.Retained austenite content, hardness and prior austenite grain size were measured. The impact toughness was determined for un-notched impact test specimens. The microstructure and fracture surfaces of impact tested samples were characterized both macroscopically and microscopically. Scanning Electron Microscopy, Light Optical Microscopy and X-Ray Diffraction measurements were used. Higher austenitization temperatures resulted in larger amounts of retained austenite and higher hardness. A higher austenitization temperature also caused coarsening of austenite grains and resulted in reduced toughness. Impact specimens austenitized and tempered at lower temperatures were found to have better toughness than those at high austenitization and tempering temperatures.
|
|
| 4. |
- Rehan, Arbab, et al.
(författare)
-
Effects of Austenitisation Temperature and Multiple Tempering on the Microstructure and Impact Toughness of a 5 wt. % Cr Cold Work Tool Steel
- 2016
-
Ingår i: 10th TOOL Conference, Tool, conference proceedings. - : 10th TOOL Conference. ; , s. 1-10
-
Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- The microstructure and properties of a 5 wt.% Cr cold work tool steel were studied after austenitisation at 1020°C, 1050°C or 1075°C followed by single, double and triple tempering treatments at 525°C. The microstructures were investigated with scanning electron microscopy and X-ray diffraction and phase transformations were studied by dilatometry. Furthermore, hardness and Charpy un-notched and V-notched impact toughness testing was performed and results were correlated to observed microstructures. With higher austenitisation temperature, the martensite and bainite start temperatures were lowered resulting in microstructures containing a higher volume fraction of retained austenite. Retained austenite transformed into martensite on cooling from the tempering temperature. Specimens that were austenitised at 1050°C or 1075°C and tempered twice contained fresh martensite. Applying a third tempering was therefore required to guarantee a fully tempered microstructure. The second tempering resulted in an increase of the un-notched impact energy while the third tempering did not have a pronounced effect. A triple tempering procedure could be preferable when austenitising at high temperatures to avoid undesirable fresh martensite in the tool microstructure.
|
|
| 5. |
- Rehan, Arbab
(författare)
-
Microstructure and mechanical properties of a 5 wt.% Cr cold work tool steel : Influence of heat treatment procedure.
- 2017
-
Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The demand for Advanced High Strength Steel (AHSS) in the automotive industry is increasing day by day. It is mainly motivated by the fact that AHSS can be used as thin sheets while having high strengths. It enables weight reduction of the automobiles which consequently increases the fuel efficiency and has proven to be less harmful to the environment. It is also expected that AHSS will have even higher strength in the near future. Cold work tools steels with 5 wt.% Cr are commonly used to process AHSS. Therefore, the tool steel must meet the challenges in the future, i.e. have even higher hardness, compressive strength and toughness. One way of increasing the mechanical properties of the tool steel is by improving the heat treatment parameters. However, it is not possible without a deeper understanding of the heat treatment process. Therefore, this work presents investigations related to phase transformations occurring in a 5 wt.% Cr cold work tool steel during heat treatment. Furthermore, the influence of austenitisation and tempering temperatures on the microstructure and mechanical properties were investigated. The studies revealed that a higher austenitisation temperature can be used to achieve a higher hardness, good compressive strength and adequate toughnessof the steel. However, too high austenitisation temperature may result inexcessive coarsening of prior austenite grains which reduced the impact toughness. It was also found that retained austenite can transform during tempering by two different mechanisms. Firstly, when tempering at 525°C, carbides precipitate in retained austenite lowering its stability and permitting a transformation to marten site on cooling. Secondly, when tempering at 600°Cfor extended holding time retained austenite isothermally transforms to ferrite and carbides. This occurs by precipitation of carbides in retained austenite followed by a final transformation to ferrite and carbides.These results were used to understand the standard tempering procedure of the 5 wt.% Cr cold work tool steel. Furthermore, alternative heat treatment procedures are discussed based on the important findings presented in this thesis.
|
|
| 6. |
- Rehan, Arbab, et al.
(författare)
-
Retained Austenite Transformation during Heat Treatment of a 5 Wt Pct Cr Cold Work Tool Steel
- 2017
-
Ingår i: Metallurgical and Materials Transactions. A. - : Springer Science and Business Media LLC. - 1073-5623 .- 1543-1940. ; 48A:11, s. 5233-5243
-
Tidskriftsartikel (refereegranskat)abstract
- Retained austenite transformation was studied for a 5 wt pct Cr cold work tool steel tempered at 798 K and 873 K (525 degrees C and 600 degrees C) followed by cooling to room temperature. Tempering cycles with variations in holding times were conducted to observe the mechanisms involved. Phase transformations were studied with dilatometry, and the resulting microstructures were characterized with X-ray diffraction and scanning electron microscopy. Tempering treatments at 798 K (525 degrees C) resulted in retained austenite transformation to martensite on cooling. The martensite start (M-s) and martensite finish (M-f) temperatures increased with longer holding times at tempering temperature. At the same time, the lattice parameter of retained austenite decreased. Calculations from the Ms temperatures and lattice parameters suggested that there was a decrease in carbon content of retained austenite as a result of precipitation of carbides prior to transformation. This was in agreement with the resulting microstructure and the contraction of the specimen during tempering, as observed by dilatometry. Tempering at 873 K (600 degrees C) resulted in precipitation of carbides in retained austenite followed by transformation to ferrite and carbides. This was further supported by the initial contraction and later expansion of the dilatometry specimen, the resulting microstructure, and the absence of any phase transformation on cooling from the tempering treatment. It was concluded that there are two mechanisms of retained austenite transformation occurring depending on tempering temperature and time. This was found useful in understanding the standard tempering treatment, and suggestions regarding alternative tempering treatments are discussed. (C) The Author(s) 2017.
|
|
| 7. |
|
|
| 8. |
- Rehan, M. Arbab, et al.
(författare)
-
EBSD Analysis of Blocky Structures in Hardened and Tempered Microstructures of a 5 wt.% Cr Cold Work Tool Steel
- 2021
-
Ingår i: Metallography, Microstructure, and Analysis. - : Springer Science and Business Media LLC. - 2192-9270 .- 2192-9262. ; 10:6, s. 862-875
-
Tidskriftsartikel (refereegranskat)abstract
- The hardened microstructure of 5 wt.% Cr cold work tool steels is mainly martensitic with significant amount of retained austenite and some undissolved carbides. Whereas the microstructure after tempering shows significant amounts of angular shaped blocky regions, which are interpretated as retained austenite or fresh martensite. The distinction between the phases is difficult due to the morphological similarities. Therefore, the blocky regions are characterized by electron backscatter diffraction to better understand the microstructures. The results showed that the characterisation of blocky structures in the microstructure of a 5 wt.% Cr cold work tool steels varied according to the austenitisation and tempering temperatures as well as the holding time at the tempering temperature. Electron backscatter diffraction of as-quenched microstructures revealed that the blocky structures were either retained austenite, fresh martensite or a combination of large part of fresh martensite with minute amounts of retained austenite. The blocky structures were entirely retained austenite after tempering at 200 °C. Tempering at 525 °C, for holding times of 0.1 or 0.5 h, more blocky regions were retained austenite contrasting to the blocky regions for holding times of 1 or 2 h, which were largely fresh martensite. It was further concluded that electron backscatter diffraction is a suitable technique for characterisation of blocky structures provided that the specimens are prepared by electro-polishing while X-ray diffraction is best suited for a bulk measurement of retained austenite.
|
|
| 9. |
- Shinde, Deodatta, et al.
(författare)
-
Improving Compositional Accuracy in APT Analysis of Carbides Using a Decreased Detection Efficiency
- 2019
-
Ingår i: Microscopy and Microanalysis. - 1435-8115 .- 1431-9276. ; 25:2, s. 454-461
-
Tidskriftsartikel (refereegranskat)abstract
- The composition of carbides in steel, measured by atom probe tomography, can be influenced by limitations in the ion detector system. When carbides are analyzed, many ions tend to field evaporate from the same region of the specimen during the same laser or voltage pulse. This results in a so-called multiple event, meaning that several ions impact the detector in close proximity both in time and space. Due to a finite detector dead-time not all ions can be detected, a phenomenon known as detector pile-up. The evaporation behavior of carbon is often different than the evaporation behavior of metals when analyzing alloy carbides, leading to preferential loss of carbon ions, and a measured carbon concentration below the expected value. This effect becomes stronger as the overall detection efficiency gets higher. Here, the detection efficiency was deliberately reduced by inserting a grid into the flight-path, which resulted in a higher and more correct carbon concentration, accompanied by an increase in the statistical uncertainty.
|
|
