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 | N O V E M B E R / D E C E M B E R 2 0 2 2 4 7 Load-to-load testing with varying parts/load in the same BIQ were done on nine loads. These yielded average measurements of surface hardness 61.4 Rc, total case 0.058 in., pitch hardness 39.5 Rc, root hardness 37.1 Rc. The surface hardness varied 1%, total case 3%, and pitch and root hardness varied 5%. This study proves consistent performance within a load and load after load in a BIQ. Surface hardness was two points greater in the gear pitch vs. root. A vacuum carburizing study was done on gears and pinions. Vacuum carburizing control is different in that it is an empirical process without in-situ carbon probes to measure carbon concentration. So, time and flow rates are controlled, using a boost-diffuse cycling for hydrocarbon media. Carbon saturation is an issue that must be controlled to prevent carbide formation in gear teeth. A pinion and 10-in. diameter gear were vacuum carburized then high-pressure gas quenched with 20 bar nitrogen. The gear root measurements had 60 Rc surface hardness, 0.038 in. (at 52.5 Rc) effective case depth. The pitch had lower surface hardness of 56 Rc but similar effective case depth of 0.040 in. A five-inch diameter gear root to pitch surface hardness was 59 Rc and 58 Rc, and effective case depth was 0.036 in. and 0.041 in. respectively. Retained austenite was 40%. The lower pitch surface hardness is opposite of atmosphere carburizing. A set of bending fatigue samples were processed with these parts and testing was being completed on these parts which would relate to gear life. A normalized furnace load was used for calculating operating costs of various batch furnace types. The load was 1000 lbs and cycle time was 3 to 4 hours to achieve a carbon profile. The comparison showed the BIQ is less expensive to operate than single chamber vacuum furnaces, Table 1. Multiple chamber vacuum carburizing furnaces proved to be very cost effective since the hot zones remain at temperature because loads are quenched in a shared oil or gas quench chamber. The flameless type BIQ had higher operating costs, but the same capacity as traditional BIQs. A closer look at a hybrid type BIQ is needed to understand the costs compared to operational advantages. The flameless type BIQ offers Fig. 2 — Workpiece definition using CHT-bf. Fig. 3 — Workpiece definition for carburizing cycle calculation using CarbTool. Fig. 4 — Carbon gradient calculation for defined workpiece using CarbTool. advantages of a clean work environment and reduced endothermic gas consumption, however introduction of ni11
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