Methods of thermo-mechanically processing tool steel and tools made from thermo-mechanically processed tool steels
A method of thermo-mechanically processing a preform composed of tool steel and a tool to modify a workpiece. The preform has a region containing austenite. The method comprises establishing the region at a process temperature between a martensitic start temperature and a stable austenitic temperature. While at the process temperature, the region is deformed to change an outer dimension and to modify the microstructure to a depth of 1 millimeter or more. The tool comprises a member composed of tool steel. The member includes a first region that extends from the outer surface to a depth of greater than 1 millimeter and a second region. The first region includes a plurality of grains having an average misorientation angle greater than about 34°, an average grain size that is at least 10% smaller than the second region, and has a different grain orientation than the second region.
1. A method of thermo-mechanically processing a preform composed of tool steel having a martensitic start temperature and a stable austenitic temperature, the preform having a first region and a second region proximate the first region, each of the first and second regions containing austenite, and the first region including an outer surface, a plurality of outer dimensions for the outer surface, and a microstructure, the method comprising:
establishing at least the first region of the preform at a process temperature between the martensitic start temperature and the stable austenitic temperature;
while the first region of the preform is at the process temperature, deforming the first region without deforming the second region to change at least one of the outer dimensions of the first region and to modify the microstructure of the first region over a depth extending from the outer surface to a depth of 1 millimeter or more beneath the outer surface; and
after the first region is deformed, cooling the first region to room temperature,
wherein the microstructure in the first region includes martensitic grains having a distribution of misorientation angles characterized by an average misorientation angle that is greater than an average misorientation angle from heat treating without deforming and differs in at least one microstructural characteristic from the second region.
2. The method of claim 1 wherein, after the first region is deformed and as a result of deforming, the outer dimensions of the first region are near net shape of a tool used in metal-forming or metal-cutting applications.
3. The method of claim 1 wherein the first region has a cross-sectional area, and the depth extends across the cross-sectional area.
4. The method of claim 1 wherein the first region has a cross-sectional area, and the change in the at least one of the outer dimensions reduces the cross-sectional area.
5. The method of claim 1 wherein the first region has a length, and the change in the at least one of the outer dimensions increases or decreases the length of the first region.
6. The method of claim 1 wherein the average misorientation angle is greater than about 34°.
7. The method of claim 1 wherein the process temperature is held isothermal while the first region is deformed.
8. The method of claim 1 wherein the process temperature is greater than an austenitic start temperature of the tool steel.
9. The method of claim 8 wherein establishing the preform at the process temperature includes heating the first region to a temperature that does not exceed the stable austenitic temperature.
10. The method of claim 1 further comprising:
prior to deforming the first region, heating the first region to a temperature above an austenitic start temperature of the tool steel and cooling the first region from the temperature above the austenitic start temperature to the process temperature.
11. The method of claim 1 wherein the process temperature is above an austenitic start temperature of the tool steel, and further comprising:
while the first region is deformed, holding the process temperature above the austenitic start temperature.
12. The method of claim 1 wherein the process temperature is between the martensitic start temperature and an austenitic start temperature of the tool steel, and further comprising:
while the first region is deformed, holding the process temperature between the martensitic start temperature and the austenitic start temperature.
13. The method of claim 1 wherein the microstructure of the first region does not recrystallize.
14. The method of claim 1 further comprising:
tempering the first region, wherein tempering includes heating the first region to a temperature that does not exceed the process temperature.
15. The method of claim 1 further comprising:
prior to deforming the first region, assembling the tool steel preform in the configuration of a shell with a core made of dissimilar steel, wherein establishing the first region at the process temperature includes establishing at least the shell at the process temperature, and, while the shell is at the process temperature, deforming at least a portion of the shell.
16. A method of thermo-mechanically processing a preform composed of tool steel having a martensitic start temperature and a stable austenitic temperature, the preform including a first region and a second region proximate the first region, each of the first and second regions containing austenite, and the first region including an outer surface, a plurality of outer dimensions for the outer surface, and a microstructure, the method comprising:
machining the preform from bulk tool steel or bar stock tool steel;
establishing at least the first region of the preform at a process temperature between the martensitic start temperature and the stable austenitic temperature;
while the first region of the preform is at the process temperature, deforming the first region to change at least one of the outer dimensions of the first region and to modify the microstructure of the first region over a depth extending from the outer surface to a depth of 1 millimeter or more beneath the outer surface; and
after the first region is deformed, cooling the first region to room temperature, the microstructure of the deformed first region having a preferred orientation of grains that provides a directionality to the microstructure of the deformed first region,
wherein deforming includes deforming the first region without deforming the second region, and after the first region is deformed and cooled to room temperature, the first region differs in at least one microstructural characteristic from the second region.
17. The method of claim 16 wherein the directionality is relative to one of the surfaces of the tool, relative to a tool axis, or relative to a second deformed region having a second directionality.
18. The method of claim 16 wherein the preferred orientation of grains in the deformed region follows the surface contour of a working surface of a tool.
19. The method of claim 18 wherein the preferred orientation follows the surface contour of the tool formed by two intersecting surfaces, the two intersecting surfaces defining an edge, the microstructure of the deformed region being substantially parallel to each of the two intersecting surfaces and transitioning from a first direction, which is parallel to one surface, to a second direction, which is parallel to the second surface, in an area proximate the edge.
20. The method of claim 16 wherein the bulk tool steel or the bar stock tool steel comprises a powdered metal.
21. The method of claim 1 wherein, prior to deforming the preform, the method further comprises:
machining the preform from bulk tool steel or bar stock tool steel.
22. The method of claim 21 wherein the bulk tool steel or the bar stock tool steel comprises a powdered metal.