March_2022_AMP_Digital

FEATURE A D V A N C E D M A T E R I A L S & P R O C E S S E S | M A R C H 2 0 2 2 3 8 tween the residual stress profiles. This, combined with the microstructural and mechanical property evaluations, bode well for fatigue performance being similar between parts subjected to the two different processes. Bend fatigue testing was conducted during this work; however, the results were impacted by surface oxidation on the DCGQ bend fatigue coupons. The oxidation was caused by the DCGQ coupons requiring open air processing with the prototype unit and was verified by SEM; produc- tion equipment can eliminate this problem by operating in a protective atmosphere. Although the DCGQ coupons were copper plated, the integrity of the copper was not maintained throughout the complete cycle, with damage likely occurring during the 1000°C austenitizing step, and subsequently worsened during the 500°C tempering cycle. The DCGQ coupons which did not fail after a few hundred cycles due to crack initiation at an oxide, would runout with stress levels comparable to HPGQ. Therefore, operat- ing DCGQ under a protective atmosphere for all processing steps should not render a decrease in in-service perfor- mance compared to HPGQ processed components. The above discussion focused on showing that cer- tain materials, particularly those which are tempered at temperatures below their MS, have the potential to acquire a similar structure and properties after a relatively slow transformation from austenite to martensite, compared to a rapid transformation from austenite to martensite. How- ever, the benefit of DCGQ is its capability to significantly reduce and control distortion of difficult-to-quench geom- etries. One such geometry was designed by DANTE Solu- tions to maximize nonuniform cooling and create large distortion. The coupon, shown in Fig. 4, was used to show the distortion reducing capabilities of DCGQ compared to HPGQ. Out-of-round distortion was the chosen mode of distortion to evaluate, due to its ease of measurement. A 50 mm eccentric bore was drilled through a disc having a 100mmdiameter and 100mmheight, creating a 6mm thin section and 44 mm thick section. A bore gauge is used to compare the measurements of the bore at the location be- tween the minimum and maximum thicknesses and at 90° to this location. The two measurements are made at five points along the height, as shown in Fig. 4. Two eccentric bore coupons were processed using HPGQ and two using DCGQ. These coupons were pro- cessed alongside the mechanical test coupons. The out- of-round distortion results for the four coupons at the five axial locations, along with the average for each cou- pon, are shown in Table 2. For the given geometry and distortion mode, DCGQ reduced the distortion by 100 µm, or 50%, compared to HPGQ. This is significant, as the DCGQ recipe used was designed for the bend fatigue coupon, which has a thinner and more uniform cross-section. The recipe was designed as such to en- sure testing was conducted using a DCGQ recipe repre- sentative of the geometry being tested. Slowing the DCGQ process down would 9 TABLE 2 — OUT-OF-ROUND DISTORTION RESULTS FOR FERRIUM C64 COUPONS Axial position Coupon DCGQ 4, mm Coupon DCGQ 5, mm Coupon HPGQ 1, mm Coupon HPGQ 2, mm 1 0.11 0.09 0.15 0.23 2 0.09 0.11 0.21 0.21 3 0.07 0.11 0.23 0.21 4 0.08 0.12 0.22 0.20 5 0.11 0.09 0.25 0.23 Average 0.092 0.104 0.212 0.216 Fig. 4 — Eccentric bore coupon used to evaluate out-of-round distortion, showing the two directions used to define out-of-round (left) and the five axial measurement points (right). 6