The conventional powder metallurgy industry identified achieving full density by a single compaction step as one of the key technical goals in the P/M Vision and Roadmap. This program will attempt to structure a systematic approach toward achieving that goal through development efforts in powder technology, compaction systems (tooling, lubrication and compaction technique) and sintering. The achievement of ultra-high green density has been demonstrated by several researchers for an elemental powder using a simple tool geometry. The goal of this program is to take those achievements and apply them to industrial alloy steel powders, using advanced commercial compaction methods and a complex part geometry, such as a gear or sprocket with a part length >25 mm with an upper limit of 100 mm. Issues that will need to be addressed include:      

Powder—low-alloy steel powder base with an equivalent hardenability to AISI 8620, need for internal lubrication vs. die-wall lubrication, type/amount of graphite addition, maximum compressibility for the powder, special lubricants (type and amount), shape/size of fine powder additions, activated additions.

Tooling—new tool designs configured for ultra-high compaction pressures, selection of tool materials, use of tool coatings, die-wall application of lubricant

Compaction systems—ultra-high pressure compaction, high-velocity compaction, warm compaction, evaluate effect of aspect ratio (length:diameter), uniform die filling,  
           
Initial studies will focus on one base powder (selected on the basis of compressibility and hardenability), three compaction approaches (warm, ultra-high-pressure, and high-velocity) three lubrication methods (die-wall, special/proprietary, and standard Acrawax) and two sintering methods (conventional - 1,140°C in 90 nitrogen/10 hydrogen and high temperature - approximately 1,280°C in vacuum and atmosphere).  The goal for Phase 1 is to reach/exceed 7.5 g/cm³ in sintered or heat treated condition; for Phase 2, to apply the concepts in Phase 1 to a multi-level part and reach 7.7 g/cm³ final density; for Phase 3, to take the technology to greater than 7.8 g/cm³ and perform complete product testing. Intermediate testing in Phases 1 and 2 will include tensile, impact, and fatigue in the carburized condition. Ultimate goal is to replace carburized 8620 wrought steel products, especially transmission gears.

Status
The preliminary program plan was approved at the spring 2004 CPMT meeting.  Independent funding sources were identified and a program leader was selected prior to the program launch during the second half of 2006. A SPFD Task Force planning meeting was held in Detroit March 24, 2006, to review the program alternatives, identify volunteers and to implement the first phase of the program. Four sub-tasks were identified and are discussed in the following sections.

Sub Task 1: High Velocity Compaction (HVC)

The gear geometry is the same gear (24 tooth, 3.00 inch P.D., 8 D.P.) that was used for the previous CPMT sponsored Single-Tooth Bend Fatigue (STBF) program.  Test gears were produced at two lengths using both conventional and warm compaction followed by conventional and high temperature sintering.         

Sub Task 2: Warm Compaction – Traditional using STBF Tool Set

Test gears were warm compacted using four different powder compositions and then both conventional and high temperature sintered.  Green densities in the 7.4 g/cm³ range were achieved.  A portion of the gears were vacuum carburized and examined for microstructure and case depth.   Following sintering and heat treatment metallurgical and mechanical properties will be measured to characterize the response of the various processing treatments.       

Sub Task 3: Warm Compaction – Effect of Part Length

An automotive 17 tooth sprocket was selected as the product application since it is currently in wrought steel.  Success of a PM sprocket demands very high wear resistance and contact fatigue strength.  Three different part lengths – 7, 11 and 22 mm were prepared.  Half were sintered in a conventional belt furnace and the other half were high temperature sintered.  Following sintering the sprockets were heat treated (case carburized and carbo-nitrided) and then performance tested using a special sprocket bench tester.
  
Sub Task 4: Ultra High Pressure Compaction – CIP

A new cold isostatic pressing (CIP) technology was used to produce test material at 110 tsi.  Two low alloy steel powders and one tool steel powder were successfully compacted and then sintered.  The low alloy steel material was machined into RCF test specimens, carburized and tested.  The tool steel material was sintered and examined for microstructure response and impact properties.