Abstract:
A method for producing compacted, fully dense articles from atomized tool steel alloy particles by placing the particles in an evacuated, deformable container, and isostatically pressing the particles at an elevated temperature to produce a precompact having an intermediate density. The precompact is heated to a temperature above the elevated temperature used to produce the precompact. The precompact is isostatically pressed to produce the fully-dense article.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to a method for producing compacted, fully-dense articles from atomized, tool steel alloy particles by isostatic pressing at elevated temperatures. 
     2. Brief Description of the Prior Art 
     In the production of powder-metallurgy produced tool steel alloys by hot isostatic compaction, it is necessary to employ sophisticated, expensive melting practices, such as vacuum melting, to limit the quantity of non-metallic constituents, such as oxides and sulfides to ensure attainment of desired properties, such as bend-fracture strength, with respect to tool steel articles made from these alloys. Practices used in addition to vacuum melting to limit the non-metallic content of the steel include using a wtundish or like practices to remove non-metallics prior to atomization of the molten steel to form the alloy particles for compacting, and close control of the starting materials to ensure a low non-metallic content therein. These practices, as well as vacuum melting, add considerably to the overall manufacturing costs for articles of this type. 
     SUMMARY OF THE INVENTION 
     It is accordingly an object of the present invention to provide a method for producing compacted, fully-dense articles from atomized tool steel alloy particles that achieve final, compacted articles of reduced oxide content without resorting to the expensive prior art practices used for this purpose. 
     In accordance with the invention, a method is provided for producing compacted, fully-dense articles from atomized tool steel alloy particles that includes placing the atomized particles in an evacuated deformable container, sealing the container and isostatically pressing the particles within the sealed container at an elevated temperature to form a precompact. The elevated temperature may be up to 1800° F. or 1600° F. This pressing may be performed in the absence of prior outgassing of the powder-filled container. The precompact is heated to a temperature above the elevated temperature used to produce this precompact and is then isostatically pressed to produce the fully-dense article. The fully-dense article may have a minimum bend fracture strength of 500 ksi after hot working. 
     The heating of the particles to elevated temperature and/or the heating of the precompact may be performed outside of the autoclave that is used for the isostatic pressing. 
     The atomized tool steel alloy particles may be gas-atomized particles which may be nitrogen gas-atomized particles. 
     Prior to isostatic pressing, the tool steel alloy particles may be provided within a sealable container. This container is evacuated to provide a vacuum therein. In addition, the deformable container is evacuated to produce a vacuum therein. The alloy particles are introduced from the evacuated container to the evacuated deformable container through an evacuated conduit. The alloy particles are isostatically pressed within the deformable container at an elevated temperature to produce the precompact having an intermediate density. The precompact is heated to a temperature above the elevated temperature used to produce the precompact and the heated precompact is isostatically pressed to produce the fully-dense article. 
     &#34;Tool steel&#34; is defined to include high speed steel. 
     The term &#34;intermediate density&#34; means a density greater than tap density but less than full density (for example up to 15% greater than tap density to result in a density of 70 to 85% of theoretical density). 
     The term &#34;outgassing&#34; is defined as a process in which powder particles are subjected to a vacuum to remove gas from the particles and spaces between the particles. 
     The term &#34;evacuated&#34; means an atmosphere in which substantially all air has been mechanically removed or an atmosphere in which all air has been mechanically removed and replaced with nitrogen. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     By way of demonstration of the invention, a series of experiments was conducted using prealloyed powder. This powder, after mechanical sizing was placed in a container that was in turn connected to a deformable container through a vacuum connection. Both containers were independently evacuated, and then the powder was loaded by use of a vibratory feeder into the deformable container. After this container was filled, it was subsequently sealed and then consolidated. Consolidation was achieved by placing the container filled with powder into a pressure vessel having internal heating capability, sealing the pressure vessel, and simultaneously raising both the temperature and pressure in the vessel to a designated high value for each--typically about 2100° F. and 14,000 psi. This process is known as hot isostatic pressing (HIP). Another consolidation method (also HIP) is to heat the sealed container externally to the designated high temperature, transfer it to a pressure vessel, seal the pressure vessel, and raise the pressure quickly to the designated high value. The method of this invention involves a novel method of consolidation which is a two step process: (1) heating the loaded container to an elevated temperature and pre-compacting it to an intermediate density followed by (2) heating it to the high temperature and hot isostatically pressing it at the temperature and pressure parameters previously described. The elevated temperature for the pre-compaction step can be up to 1800° F. This pre-compaction step increases the density of the powder, but not to full density. 
     The tested alloys were designated as CPM 10 V (10 V), CPM M4 High Carbon (M4HC), and CPM M4 High Carbon with Sulfur (M4HCHS). 
     
                       TABLE 1______________________________________Composition of Alloys Tested (Balance Fe)  Alloy    C       Mn   Si    S    Cr   Mo   W    V______________________________________10 V   2.45    0.50   0.90  0.07 5.25 1.30 --   9.75  M4HC   1.40    0.30     0.30     0.05     4.00    5.25   5.75    4.00                                            M4HCHS 1.42   0.70                                           0.55     0.22      4.00                                            5.25   5.75   4.00______________________________________ 
    
     All tests started with containers having a minimum diameter of 14 inches, and were conducted on material that had been hot worked with a reduction in area of at least 75%. M4 types were solution heat treated at 2200° F. and triple tempered at 1025° F. The data are presented by powder type, alloy, and consolidation method. The conventional consolidation method in which the temperature and pressure are simultaneously raised is designated as &#34;CCMD HIP.&#34; The process of externally heating, transferring to the pressure vessel, and raising the pressure is designated at &#34;CSMD HIP.&#34; The method of the invention as described in the preceding paragraph is designated as &#34;WIP/HIP.&#34; 
     Table 2 presents data from trials of the alloy designated as M4HCHS. The practice used to produce this alloy powder comprised melting raw materials in an induction furnace, adjusting the chemistry of the molten alloy prior to atomization, pouring the molten alloy into a tundish with a refractory nozzle at the base of the tundish, and subjecting the liquid metal stream from that nozzle to high pressure nitrogen gas for atomization thereof, to produce spherical powder particles. 
     
                       TABLE 2______________________________________M4HCHS                Bend Fracture Results                                   Max.,  Trial  Powder    Consolidation             Average   Min.  Number        Size      Method                Tests  (ksi)  (ksi)______________________________________MFG 17  -16 Mesh  CCMD HIP   6    434   458,382  MFG 18   -l6 Mesh    CCMD HIP       6    475    530,433  MFG 43   -16 Mesh    CCMD HIP       6    541    581,496  MFG 44   -16 Mesh    CCMD HIP       5    548    594,488  MFG 40   -35 Mesh    CCMD HIP       5    576    597,554  MFG 41   -35 Mesh    CCMD HIP       6    534    605,380  MFG 42   -35 Mesh    CCMD HIP       3    461    536,318  MFG 69   -35 Mesh    CCMD HIP       15   617    674,567  MFG 70   -35 Mesh    CCMD HIP       15   589    632,467  MFG 61   -35 Mesh    CCMD HIP        6   506    570,455  MFG 71   -35 Mesh    CCMD HIP       15   463    551,360  MFG 72   -35 Mesh    CCMD HIP       12   455    550,361  MFG 105  -35 Mesh    CCMD HIP       15   517    596,400  MFG 106  -35 Mesh    CCMD HIP       15   484    583,441  MFG 107  -35 Mesh    CCMD HIP       15   505    574,428  MFG 108  -35 Mesh    CCMD HIP       13   506    596,405  MFG 109  -35 Mesh    CCMD HIP       75   559    630,422  MFG 73   -35 Mesh*    CCMD HIP       15   454    530,228  MFG 105A -35 Mesh*    CCMD HIP       15   543    579,496  MFG 106A -35 Mesh*   CCMD HIP        15   495    565,418  MFG 107A -35 Mesh*   CCMD HIP         15   449    530,393  MFG 72   -35 Mesh**   CCMD HIP        15   467    527,386  MFG 72   -35 Mesh**   CCMD HIP         14   459   600,350  MFG 72   -35 Mesh**   CCMD HIP       15    450  543,330  MFG 66   -35 Mesh     WIP/HIP        15    439   528/361  MFG 67   -35 Mesh     WIP/HIP        15    429   541,299  MFG 68   -35 Mesh     WIP/HIP        15    488   577,344  MFG 69   -35 Mesh     WIP/HIP        15    597   645,525  MFG 70   -35 Mesh     WIP/HIP        30    569   594,459  MFG 105  -35 Mesh     WIP/HIP        15    466   539,253  MFG 106  -35 Mesh     WIP/HIP        15    446   525,353  MFG 107  -35 Mesh     WIP/HIP        15    404   504,245  MFG 108A -35 Mesh     WIP/HIP        29    448   562,322  MFG 108B -35 Mesh     WIP/HIP        30    443   518,269  MFG 109  -35 Mesh     WIP/HIP        60    525   593,431______________________________________ -35 Mesh*: Finer than normal distribution. -35 Mesh**: Various mixtures of -35 mesh and -100 mesh powder. 
    
     As may be seen from the Table 2 data, product that was initially screened to -35 mesh and was consolidated by the CCMD HIP showed individual test results of bend fracture strengths up to 674 ksi. The averages ranged from a low of 449 ksi to a high of 617 ksi. The minimum bend fracture strength test results are not characteristics of the practice. These low results were caused by large exogeneous inclusions present at the bend fracture surfaces. 
     The exogenous inclusions were identified as either slag or refractory particles. The slag originated from oxidized material as a result of exposure to air during melting. The refractory originated from erosion during the melting and the pouring of the alloy prior to atomization. They thus originated during melting and it is their presence that caused the low bend fracture results. 
     These low results are caused, therefore, not by the consolidation practice, but by the melting practice, and are not characteristic of the properties typically resulting from use of the consolidation practice. The maximum bend fracture strength of the product consolidated by the WIP/HIP method was 645 ksi, which is only slightly below the maximum value from the CCMD HIP. The average bend fracture strength values using WIP/HIP ranged from a low of 404 ksi to a high of 597 ksi. There is some difference between the CCMD HIP and the WIP/HIP process, but it is quite small. The low minimum values are caused by melting, not consolidation, so it is the high value of the averages that is most significant. Because productivity was much greater using the WIP/HIP process, and the capital equipment necessary to practice it costs much less than that required for CCMD HIP, there is an economic advantage to the method in accordance with the invention. Both the maximum values and the average bend fracture strengths of the two consolidation methods are comparable. These data clearly show that the WIP/HIP consolidation method yielded high bend fracture strength results. 
     A smaller number of trials was run on M4HC produced by the same practice as used in the production of M4HCHS. Results from these trials are shown in Table 3. 
     
                       TABLE 3______________________________________M4HC                Bend Fracture Results                                   Max.,  Trial  Powder    Consolidation             Average   Min.  Number  Size      Method         Tests  (ksi)  (ksi)______________________________________MFG 33  -35 Mesh  CCMD HIP   6    622   666,589  MFG 34        -35 Mesh        CCMD HIP        6            606                                   647,581  MFG 35        -35 Mesh        CCMD HIP        6            622                                   639,577  No Number     -35 Mesh        CCMD HIP        6            708                                   732,658  MFG 36        -35 Mesh        CCMD HIP        6            612                                   627,595  MFG 37         -35 Mesh        CCMD HIP        6            615                                   653,550  MFG 38        -35 Mesh        CCMD HIP        4             663                                   695,607  MFG 73      -35 Mesh*        CCMD HIP        15           454                                   530,228  MFG 37      -35 Mesh*         WIP/HIP        3            580                                   615,493______________________________________ 
    
     Two observations can be made: (1) the bend fracture strength of the lower sulfur (M4HC) material was significantly greater than for the high sulfur (M4HCHS) material, regardless of the consolidation method, and (2) the average bend fracture strength of the WIP/HIP material, while well above 500 ksi, was below that consolidated by CCMD HIP. 
     Table 4 shows the data from trials of 1 V alloy produced by the same practice as M4HCHS. 
     
                       TABLE 4______________________________________10 V                Bend Fracture Results                                   Max.,  Trial  Powder    Consolidation             Average   Min.  Number        Size      Method                Tests  (ksi)  (ksi)______________________________________MFG 7   -35 Mesh  CCMD HIP   48   572   651,331  MFG 8      -35 Mesh      CCMD HIP             48          578                                   651,357  MFG 45     -35 Mesh      CCMD HIP             18          562                                   656,348  MFG 46     -35 Mesh      CCMD HIP             18          563                                   644,361  MFG 47     -35 Mesh      CCMD HIP             12          550                                   640,386  MFG 48     -35 Mesh      CCMD HIP             12          558                                   645,402  MFG 52     -35 Mesh      CCMD HIP             12          602                                   649,551  MFG 53     -35 Mesh      CCMD HIP             24          615                                   663,552  MFG 55     -35 Mesh      CCMD HIP             11          616                                   663,552  MFG 61     -35 Mesh*     CCMD HIP             12          587                                   663,552  MFG 63     -35 Mesh*     CCMD HIP             15          550                                   621,385  MFG 65     -35 Mesh*     CCMD HIP             3           610                                   646,592  MFG 63     -35 Mesh*     WIP/HIP              20          540                                   612,409  MFG 49     -35 Mesh      CSMD HIP             6           456                                   523,405______________________________________ 
    
     These results show that WIP/HIP consolidation gave average bend fracture strengths for this alloy that are lower than the CCMD HIP consolidation, but significantly above the CSMD HIP. The values below 500 ksi with the CCMD HIP or WP/HIP consolidation had large exogenous inclusions in the fracture surface, as a result of the melting practice. The maximum strength values showed that the WIP/HIP method gave strengths about 50 ksi lower than CCMD HIP, but still well above the 500 ksi minimum. 
     All of the WIP/HIP trials discussed above used a temperature of 1400° F. for the pre-compacting temperature. This temperature was chosen based on work that is described hereafter. In all of the above disclosed cases, the loaded compacts were externally heated and transferred to the pressure vessel and the pressure was quickly raised to 11,000 psi. After this pre-compaction step, the compacts were each transferred to a furnace operating at 2150° F. equalized, and then transferred to the pressure vessel. 
     The vessel was sealed and quickly pressurized to 14,000 psi. The consolidated compacts, regardless of the consolidation method, were all thermo-mechanically processed to about 85% reduction from their original size before the bend fracture strength was tested. 
     Experimental work was carried out on the effect of heating at various temperatures prior to conventional consolidation (CCMD HIP). M4HCHS powder screened to -35 mesh was loaded into 5&#34; diameter cans, sealed, and heated for five hours at temperatures ranging from 1400 to 2185° F. After holding at this temperature, the compacts were given conventional (CCMD HIP) consolidation with final temperature and pressure of 2185° F. and 14,000 psi, respectively. Bend fracture strength tests were run in the as-HIP condition, and after hot working with an 82% reduction in area from the original compact size. Test results are given in Table 5. 
     
                       TABLE 5______________________________________Bend Fracture Test Results on Pre-Heated Powder      Pre-Heat  As-HIP  Powder     Temperature      Bend Fracture  Hot-Worked Bend Fracture                          Source     (                         ° F.)        (ksi)                          (ksi)______________________________________A      No Hold   492        603                                 1400                  501                                  602                                 1600                  452                                  605                                 1800                  453                                  601                                 2000                  429                                  579                                 2185                  367                                  582  B                No Hold                                  529                                  647                                 1400                  547                                  643                                1600                   426                                  642                                 1800                  446                                  601                                 2000                  405                                  578                                 2185                  362                                  567______________________________________ 
    
     These results show that when unconsolidated power was held at temperature above 1400° F. bend fracture strenghts in the as-HIP condition were lowered. When tested after an 82% reduction by hot working, bend fracture strenghts were not lowered until the powder is held at temperatures in excess of 1600° F. As a result of these data, all heating for the pre-compaction was done at 1400° F. as previously stated. 
     To determine the reason for this degradation in bend fracture strength, a determination had to be made as to whether heating at these different temperatures had any effected on the sulfide and oxide distribution, both in the as-HIP condition and after hot working. The results of this examination are given in Table 6. 
     
                       TABLE 6______________________________________Sulfide Distribution on Pre-Heated Powder     Pre-Heat     Sulfide Distribution                             Sulfide Distribution  Powder  Temperature          As-HIP                Hot WorkedSource (° F.)              Area    Max.Size                             Area  Max. Size______________________________________ B    No Hold      225     3.61    253  6.56                              1400         152          2.59                                       124           5.85                             1600         185         3.38                                     343           13.34                            1800         315          4.19                                     402           5.76                             2000         540          5.06                                      656           9.43                2185                     993          10.78                                     1071           18.53______________________________________ 
    
     These data show that if the pre-heat temperature is 1600° F. or higher, the total sulfide area increased, the increase was greater with a higher hold temperature. This is shown for both the as-HIP as well as the hot worked condition. It is well known that larger inclusions as well as larger total area of inclusions cause a degrease in bend fracture strength. Microstructural examination of the effect of pre-heat temperature on oxide growth showed no apparent increase in the size of the oxides for pre-heat temperatures up to 2000° F. but at pre-heat temperatures above 1600° F. there was a noticeable outlining of the prior particle boundaries indicating the beginning of an increased concentration of oxides. For these reasons, all production trial compacts were pre-heated at 1400° F. but could have been pre-heated up to 1600° F. without any detrimental affect. 
     Other embodiments of the present invention will be apparent to those skilled in the art from consideration of the specification and practice of the invention disclosure 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.