Patent Application
20080078475
Table 1 discloses the chemical compositions that were examined experimentally and that led to the alloy of the invention that achieves an improved combination of toughness and wear resistance. The chemical compositions of Alloy A and Alloy B are included for comparison purposes.
Prealloyed cold-work tool steels of the reported chemical compositions, except for alloy B, were melted in a nitrogen atmosphere, atomized by nitrogen gas, and hot-isostatically-pressed (HIP).
The alloy of the invention is designed to have approximately the equivalent matrix chemical compositions and the volume fractions of MC primary carbides as Alloy A. The key improvement over Alloy A in terms of toughness characteristics is due to the discovery that the size distribution of the Nb-rich MC primary carbides in the alloy of the invention is shifted toward smaller primary carbides compared to the size distribution of the V-rich MC primary carbides in Alloy A (
Approximately 50 lbs of the alloy of the invention (Alloy LGA) was melted and atomized on the Laboratory Gas Atomizer (LGA) having a capacity of 50 lbs., and about 650 lbs of the alloy of the invention (Alloy PGA) was melted and atomized on the Pilot Gas Atomizer (PGA), having a capacity of 800 lbs., at Crucible Research. The chemical analyses of the two heats are given in Table 1.
With respect to the various alloying elements in the alloy of the invention, the following applies:
Carbon is present in an amount of at least 0.5%, while the maximum content of carbon may amount to 1.2%, and preferably in the range of 0.75-0.85%. It is important to carefully control the amount of carbon in order to obtain a desired combination of toughness and wear resistance, as well as to avoid forming unduly large amounts of retained austenite during heat treatment.
Nitrogen is present in an amount of 0.02-0.20%, and preferably in the range of 0.08-0.14%. The effects of nitrogen in the alloy of the invention are rather similar to those of carbon. In tool steels, where carbon is always present, nitrogen forms carbonitrides with vanadium, niobium, tungsten, and molybdenum.
Silicon may be present in an amount of 0.3-1.3%, and preferably in the range of 0.5-1.1%. Silicon functions to deoxidize the prealloyed materials during the melting phase of the gas-atomization process. In addition, silicon improves the tempering response. Excessive amounts of silicon are undesirable, however, as it decreases toughness and promotes the formation of ferrite in the microstructure.
Manganese may be present in an amount of up to 1%, and preferably up to 0.5%. Manganese functions to control the negative effects of sulfur on hot workability. This is achieved through the precipitation of manganese sulfides. In addition, manganese improves hardenability and increases the solubility of nitrogen in the liquid prealloyed materials during the melting phase of the gas-atomization process. Excessive amounts of manganese are undesirable, however, as it can lead to the formation of unduly large amounts of retained austenite during the heat treatment.
Chromium is present in an amount of 6.0-9.0%, and preferably in the range of 7.0-8.0%. The main purpose of chromium in cold-work tool steels is to increase hardenability and secondary-hardening response.
Molybdenum is present in an amount of 0.6-2.0%, and preferably in the range of 1.0-1.5%. Like chromium, molybdenum increases hardenability and secondary-hardening response of the alloy of the invention. Excessive amounts of molybdenum, however, reduce hot workability.
Tungsten is present in an amount of 0.5-3.0%, and preferably in the range of 1.3-1.8%. Like chromium and molybdenum, tungsten increases hardenability and secondary-hardening response of the alloy of the invention. In cold-work tool steels, tungsten behaves in a similar manner as molybdenum, with which it is interchangeable on an atomic basis; approximately 1.9 wt. % W has the same effect as 1 wt. % Mo.
Vanadium is present in an amount of 0.2-2.0%, and preferably in the range of 0.5-1.0%. Vanadium is critically important for increasing wear resistance. This is achieved through the precipitation of MC type primary carbonitrides.
Niobium is present in an amount of 1.5-4.0%, and preferably in the range of 2.25-2.75%. Every percent of niobium is equivalent to the amount of vanadium calculated as follows:
% V=(50.9/92.9)×% Nb
where 50.9 and 92.9 are atomic weights of vanadium and niobium, respectively. In cold-work tool steels, niobium and vanadium are equivalent elements with respect to wear resistance.
Powder of the alloy of invention produced on Laboratory Gas Atomizer (Alloy LGA) and on Pilot Gas Atomizer (Alloy PGA) was containerized into 4.5-5″ OD containers and was hot isostatically pressed (HIP), and then forged into a 3″×1″ bar, Alloy LGA, or a 3″×1.25″ bar, Alloy PGA.
The heat-treatment response of Alloy LGA (the alloy of the invention) is given in Table 2. The following two austenitization temperatures were selected: 1950° F. and 2050° F. The results are comparable to those of the Alloys A and B.
The longitudinal and transverse bend fracture strength (BFS) and Charpy C-notch (CCN) impact toughness of the 3″×1″ and 3″×1.25″ forged bars of the alloy of the invention were also evaluated. The following two austenitization temperatures were selected: 1950° F. and 2050° F. The CCN and BFS specimens were tempered at 1025° F. for 2 hours+2 hours.
A 6.35 mm×6.35 mm×55 mm specimen, supported by two cylinders, is used in the three-point BFS test. The distance between the supporting cylinders is 25.4 mm. The third cylinder is used to apply a load until the BFS specimen fractures, the applied load being equidistant from the either supportive cylinders. The load at which the BFS specimen breaks is used to calculate the numerical value of bend fracture strength.
The geometry of a specimen used to measure Charpy C-notch impact toughness is similar to that used to measure Charpy V-notch impact toughness: 10 mm×10 mm×55 mm. The radius and the depth of the C-notch are 25.4 mm and 2 mm, respectively.
The BFS and CCN results obtained from Alloy LGA and Alloy PGA, and Alloys A and B are given in Table 3 and Table 4, respectively. The alloy of the invention demonstrated superior toughness characteristics compared to the benchmark alloys, as measured with bend fracture strength and Charpy C-notch impact toughness.
Finally, four heat-treated pin-abrasion wear-resistance specimens were tested from the alloy of the invention. Two specimens were machined from the Alloy LGA and two specimens were machined from the Alloy PGA. The austenitization temperatures of 1950° F. and 2050° F. were selected. After quenching in oil, all the specimens were tempered at 1025° F. for 2 hours+2 hours. The pin-abrasion wear resistance test results are given in Table 5. The pin abrasion test results for Alloy A and Alloy B are included for comparison.
The size distribution of primary carbides in the alloy of invention and Alloy A was measured using an automatic image analyzer. The diameter of carbides was measured in fifty random fields examined at an optical magnification of 1000×. The count of primary carbides (per square millimeter) of various sizes in the alloy of the invention and Alloy A is plotted in
The graph in
Other embodiments of the invention will be apparent to those skilled in the art from consideration of the specification and practice of the invention disclosed herein. It is intended that the specification and examples be considered as exemplary only, with a true scope and spirit of the invention being indicated by the following claims.
