Advancements in Precision Carburizing by F. Otto and D. Herring

ADVANCEMENTS IN PRECISION CARBURIZING
OF NEW AEROSPACE AND MOTORSPORTS MATERIALS

Demand for vacuum carburizing systems equipped with high pressure gas quenching, oil quenching, or both has quadrupled since 2000. Commercial heat treating has seen the second largest increase in the number of installed low pressure vacuum carburizing units.

Frederick J. Otto**
Midwest Thermal-Vac (MTV) Inc.
Kenosha, Wis.

Daniel H. Herring**
The Herring Group Inc. Elmhurst, Ill.

** Member of ASM International and member, ASM Heat Treating Society

Progress continues to be made in low pressure vacuum carburizing for advanced applications in the aerospace and motorsports markets since 2005 [1], particularly in the development of carburizing cycles for new materials and materials previously not vacuum carburized. Materials successfully vacuum carburized reported in 2005 included:
• Aubert & Duval X12 VDW and XD15NW
• Carpenter Technology Corp. Pyrowear 53 and 675
• Böhler-Uddeholm N360 Iso Extra, N695, R250, R350
• Questek Innovations Ferrium C61, CS62, C69, M60S, and S53
• Timken Company CSS-42L, CSB- 50NIL, CBS-600, and BG42VIM/ VAR
• Atlas Specialty Steels BS970 EN30B
New grades carburized since that time include:
• CarTech AerMet 100
• Timken CBS 223, AF1410, HY180, HP-9-430, and 300M
• VSG Essen Cronidur 30
• Teledyne Corp. VascoMax C-250, C-300, and C-350
In addition, the range of carburizing case depths for all grades has been extended extensively in the past two years (Table 1).

What Has Been Achieved
MTV studied critical process parameters and their interactions including:
• Required temperature uniformity
• Methods of carbide control
• Avoidance of carbide networks and necklace formation
• Surface carbon control • Achievable case uniformity
• Maximum achievable core properties
• Quenching parameters for distortion sensitive and distortion prone part geometry

Effective case depths have been extended to 0.080-0.120 in. (2 0-3.0 mm), and beyond, without significant sacrifice of microstructure. The range of carburizing temperatures now includes the use of high temperature techniques.

Table 1 — Production cycle process parameter interaction

Material	Achievable ECD, in. (mm)	Temp. range, °F (°C)	Core hardness, HRC	Case hardness, HRC	Surface carbon, %
X13VDW	0.015-0.100 (0.38-2.55)	1675-1900 (915-1040)	43-46	59-63	0.86-1.05
XD15NW	0.015-0.065 (0.38-1.65)	1700-1950 (925-1065)	36-55	58-64	0.72-0.92
Pyrowear 675	0.015-0.095 (0.38-2.40)	1600-1925 (870-1050)	39-43	62-66	0.875-1.2
Pyrowear 53	0.015-0.085 (0.38-2.15)	1600-1850 (870-1010)	3640	59-63	0.55-0.72
AerMet 100	0.015-0.060 (0.38-1.53)	1600-1650 (870-900)	52-55	58-63	0.57-0.79
N360 Iso Extra	0.015-0.035 (0.38-0.90)	1675-1925 (915-1050)	56-58	63-67	0.60-0.72
N695	0.025-0.55 (0.65-1.40)	1650-2020 (900-1105)	55-60	62-65	0.56-0.69
R250	0.020-0.045 (0.50-1.15)	1680-1825 (915-1000)	50-52	62-64	0.65-0.78
R350	0.025-0.060 (0.65-1.53)	1680-2010 (915-1100)	52-55	63-66	0.66-0.82
C61	0.015-0.100 (0.38-2.55)	1900-2025 (1040-1110)	48-52	60-70	0.42-0.53
CS62	0.015-0.045 (0.38-1.15)	1900-2025 (1040-1110)	49-51	60-62	0.40-0.48
C69	0.015-0.100 (0.38-2.55)	1850-1925 (1010-1050)
CSS-42L	0.015-0.075 (0.38-1.90)	1650-2050 (900-1120)	28-45	58-69	0.55 – 0.70
CSB-50NIL	0.015-0.060 (0.38-1.53)	1650-2050 (900-1120)	47-50	58-63	0.62-0.83
CBS-600	0.015-0.075 (0.38-1.90)	1525-1750 (830-955)	40-43	58-64	0.78-1.00
BG42	0.015-0.065	1650-1850	44-47	61-64	0.65-0.88
AF1410	0.015-0.075 (0.38-1.90)	1525-1650 (830-900)	47-49	58-62	0.84-1.05
HY180	0.015-0.075 (0.38-1.90)	1500-1550 (815-845)	44-47	58-62	0.82-1.10
HP-9-420	0.015-0.045 (0.38-1.15)	1500-1600 (815-870)	39-42	56-59	0.95-1.25
HP-9-430	0.015-0.075 (0.38-1.90)	1525-1700 (830-925)	47-50	58-62	0.85-1.05
300M	0.015-0.045 (0.38-1.15)	1600-1700 (870-925)	53-55	58-62	0.80-0.92
BS 970 EN30B	0.015-0.055 (0.38-1.40)	1550-1750 (845-955)	52-54	58-62	0.65-0.825
VSG Essen Cronidur 30	0.020-0.055 (0.50-1.40)	1600-1890 (870-1030)	55-57	62-66	0.58-0.77
VascoMax C-250	0.015-0.075 (0.38-1.90)	900-1525 (480-830)	48-50	57-61	0.82-0.95
VascoMax C-300	0.015-0.075 (0.38-1.90)	900-1525 (480-830)	51-54	58-62	0.82-0.97
VascoMax C-350	0.015-0.075 (0.38-1.90)	900-1525 (480-830)	55-59	59-62	0.83-1.02

Notes: Maximum case depth variation (part-to-part) within a load can be held to ±0.0025 in. (±0.0635 mm), and is routinely done so for aerospace and most motorsports applications. Temperature uniformity is held to ±4.5°F (±1.5°C). Program recipes establish hydrocarbon gas type, flow, pressure, gas injection (boost) duration, incremental diffusion duration, and final diffusion length as a function of material chemistry, load surface area, and customer specification. Vacuum carbonitriding case depths from 0.0003-0.025 in. (0.0076-0.635 mm) can be achieved in commercial practice.

Progress continues to be made in low pressure vacuum carburizing for advanced aero and motorsports applications.

Improved mechanical properties such as fracture toughness and enhanced corrosion resistance have been achieved in many new aero and motorsport materials using precision vacuum carburizing.

All these materials required extensive development testing to produce custom designed recipes to optimize cycle parameters. Some of the new techniques developed to achieve these results within an existing recipe required changes to temperature, flow rate, pressure, and hydrocarbon type. Also, high pressure gas quench-ing methods involved the use of various gas mixtures (nitrogen, nitrogen/hydrogen, and carbon dioxide/helium) in addition to vacuum oil quenching.

Application to Advanced Materials
The list of acceptable materials for vacuum carburizing is growing (Table 2) and now includes over a dozen new grades. Improved mechanical properties such as fracture toughness and enhanced corrosion resistance compared with published literature values has been achieved, especially for elevated temperature service applications in the range of 600 to 950°F (315 to 510°C) and higher (Table 3). Many of these materials are similar in chemistry to those of stainless and tool steels, and now ultrahigh strength steels have been added to the list. Carburizing at temperatures higher than 1850°F (1010°C) is a focus of active research and some commercialization. Program recipes at MTV use temperatures as high as 2200°F (1205°C) in commercial runs.

Control of Microstructure
The challenge today is to produce an optimized part microstructure (Figs. 1- 4) while using the highest possible carburizing temperature. In some cases, cycle times have been reduced by 33 to 50% over those of atmosphere carburizing; processes requiring several hundred hours are no longer necessary. Multiple short duration boost cycles (sometimes in the range of seconds) and long diffusion times (sometimes in the range of hours) provide just enough carbon to the surface of the part while avoiding the formation of retained austenite, carbide networks, and necklaces.

Table 2 — Chemical composition of advanced carburizing alloys

						Content, wt%
	C	Mn	Cr	Ni	Mo	Si	V	Co	Nb	Ti	Al	Zr	B
XD15NW	0.37	—	15.5	0.20	1.80	—	0.30	—	—	—	—	—	—
X13VDW	0.12	—	11.5	2.50	1.60	—	0.03	—	—	—	—	—	—
Pyrowear 675	0.07	0.65	13.0	2.60	1.80	0.40	0.60	5.40	—	—	—	—	—
Pyrowear 53	0.10	0.35	1.00	2.00	3.25	1.00	0.10	—	—	—	—	—	—
AerMet 100	0.23	—	3.10	11.10	1.20	—	—	13.4	—	—	—	—	—
N360 Iso Extra	0.33	0.50	15.0	0.40	1.00	—	—	—	—	—	—	—	—
N695	1.05	0.20	17.0	—	0.50	0.50	—	—	—	—	—	—	—
R250	0.83	0.70	4.00	—	4.30	0.20	1.10	—	—	—	—	—	—
R350	0.14	0.30	4.25	3.50	4.30	0.18	—	—	—	—	—	—	—
C61	0.16	—	3.5	9.5	1.1	—	0.08	18.0	—	—	—	—	—
CS62	0.08	—	9.0	1.5	—	—	0.2	15.0	—	—	—	—	—
C69	0.090.11	—	5.0-5.2	2.9-3.1	2.4-2.6	—	0.015-0.025	27.8-28.2	—	—	—	—	—
CSS-42L	0.12	—	14.0	2.00	4.75	—	0.60	12.5	0.02	—	—	—	—
CSB-50NIL	0.13	0.25	4.20	3.40	4.25	0.20	1.20	—	—	—	—	—	—
AF1410	0.13-1.17	0.10	1.8-2.2	9.5-10.5	1.0	0.10	—	13.5-1 4.5	—	—	—	—	—
CBS 223	0.15	0.40	4.95	0.10	1.45	0.90	—	1.55	—	—	0.05	—	—
CBS-600	0.19	0.60	1.45	—	1.0	1.1	0.06	—	—	—	0.06	—	—
BG42	1.15	0.50	14.5	—	4.00	0.30	1.20	—	—	—	—	—	—
HP 9-4-30	0.29-0.34	0.35	0.90-1.10	8.00-9.50	—	0.20	—	4.25-4.75	—	—	—	—	—
HY-180	0.13	0.10	2.0	10.0	1.0	0.05	—	—	—	—	—	—	—
300M	0.38-0.46	0.6-0.9	0.7-0.95	1.65-2.0	0.30-0.65	1.45-1.8	0.05 min	—	—	—	—	—	—
835M30 EN30B ANN	0.26-0.34	0.45-0.70	1.10-1.40	—	0.20-0.35	0.10-0.35	—	—	—	—	—	—	—
Cronidur 30	0.31	—	17.0	0.38	1.02	0.55	—	—	—	—	—	—	—
VascoMax C-250	0.03 max	0.10 max	—	18.5	4.80	0.10 max	—	7.50	0.40	0.10	—	0.01	0.003
VascoMax C-300	0.03 max	0.10 max	—	18.5	4.80	0.10 max	—	9.00	0.60	0.10	—	0.01	0.003
VascoMax C-350	0.03 max	0.10 max	—	18.5	4.80	0.10 max	—	12.00	1.40	0.10	—	0.01	0.003

Aerospace Industry
Improved performance demands in aircraft and rotorcraft are not only pushing advanced materials, but also are forcing prime contractors and their suppliers to use low pressure vacuum carburizing in more challenging applications. Typical commercial and military applications (Fig. 5 and 6) of low pressure vacuum carburizing for aerospace vehicles include such items as braking systems, actuator systems, flight controls and guidance systems, hydraulic power plants, and landing gearboxes, and involves components such as bearings, ball screws and nuts, planetary gears, pinions, and shafts. The commercial heat treating in-dustry is beginning to share its wealth of knowledge with respect to acknowledged successes and lessons learned. Improved cleaning tech-nology (including rinsing and drying), as well as the elimination of manufacturing steps such as pre-ox-idation, are helping to improve quality. Lead times are being reduced and process control improvements are better documented along with maintenance records and up-time re-liability. What is needed going for-ward are better specifications and rules for the use of high pressure gas quenching.

Table 3 — Achievable performance enhancements

Material	Surface hardness, HRC	UTS, MPa (ksi)	YS 0.2% offset, MPa (ksi)	Charpy V-notch impact energy, N • m• lbf)	Fracture toughness, MPa (ksi)	Tempering temp., °C (°F)
Carburizing (vacuum)
4340	53	1,979 (287)	1,862 (270)	20.3 (15)	331 (48)	205 (400)
4340	46	1,496 (217)	1,365 (198)	29.8 (22)	489 (68)	425 (800)
4340	40	1,241 (180)	1,158 (168)	47.5 (35)	689 (100)	540 (1000)
H11	56	2,006 (291)	1,675 (243)	20.3 (15)	331 (48)	540 (1000)
H11	48	1,641 (238)	1,413 (205)	27.1 (20)	427 (62)	580 (1075)
H11	44	1,427 (207)	1,276 (185)	31.2 (23)	483 (70)	595 (1100)
300M	56	2,344 (340)	1,241 (180)	17.5 (13)	310 (45)	95 (200)
300M	54	2,137 (310)	1,655 (240)	21.7 (16)	345 (50)	205 (400)
300M	45	1,793 (260)	1,482 (215)	13.6 (10)	235 (34)	425 (800)
300M	40	1,586 (230)	1,358 (197)	42.0 (31)	689 (100)	540 (1000)
EN30B	52	1.793 (260)	1489 (216)	36,6 (27)	565 (82)	205 (400)
EN30B	40	1.400 (203)	1289 (187)	44,7 (33)	689 (100)	540 (1000)
Carburizing (vacuum)
300M	63/65	2,344 (340)	1,241 (180)	17.5 (13)	310 (45)	95 (200)
300M	61/63	2,137 (310)	1,655 (240)	21.7 (16)	345 (50)	205 (400)
AerMet 100	61/63	1,931 (280)	1,724 (250)	56.9 (42)	827 (120)	480 (900)
AerMet T	61/63	1,965 (285)	1,724 (250)	33.9 (25)	558 (81)	480 (900)
AerMet 310	61/63	2,172 (315)	1,896 (275)	27.1 (20)	448 (65)	480 (900)
AF1410	59/62	1,813 (263)	1,586 (230)	80.0 (59)	1.255 (182)	495 (925)
AF1410	58/61	1,710 (248)	1,551 (225)	93.6 (69)	1.469 (213)	510 (950)
EN30B	62/65	1,793 (260)	1,489 (216)	36.6 (27)	565 (82)	95 (200)
EN30B	58/62	1,400 (203)	1,289 (187)	44.7 (33)	689 (100)	205 (400)
Pyrowear 53	59/63	2,000 (290)	1,620 (235)	38.0 (28)	517 (75)	315 (600)
VascoMax C-250	61/63	1,792 (260)	1,758(255)	50.2 (37)	786 (114)	480 (900)
VascoMax C-300	61/63	2,027 (294)	1,999 (290)	28.0 (28)	517 (75)	480 (900)
VascoMax C- 350	61/63	2,413 (350)	2,344 (340)	13.5 (10)	234 (34 )	480 (900)

chart 1
Fig. 1 — (a) AerMet 100 vacuum carburized to 0.055 in. (1.40 mm) ECD: 58 HRC @ 0.045 in. (1.15 mm); 60-62
HRC surface hardness; (b) microindentation hardness traverse. 1250X

chart 2
Fig. 2 — (a) AF1410 vacuum carburized to 0.055 in. (1.40 mm) ECD: 60 HRC @ 0.032 in. (0.82 mm); 60-62 HRC
surface hardness; (b) mi0.005 croindentation hardness traverse. 1250X

chart 3
Fig. 3 — (a) CBS-223 vacuum carburized to 0.057 in. (1.45 mm) ECD: 63-65 HRC; (b) microindentation hardness
traverse. 1250X

chart 4
Fig. 4 — (a) X13 VDW vacuum carburized to 0.035 in. (0.89 mm) ECD: 63-65 HRC; (b) microindentation hardness
traverse. 1250X

Fig. 5 — Hydraulic system Pyrowear 675 bearings.

Fig. 6 — Acutator system X13VDW ballscrews and nuts, gears, and planetary plates.

Engine Fig. 7 — Top fuel dragster engine.

Table 4 — Final 2006 NASCAR/Nextel and Busch Series Racing Results
Nascar results

Motorsports Update
MTV’s continued participation in motorsports (racecars, dragsters, off-road vehicles, trucks, and top fuel funny cars) has helped produce im-pressive results (Table 4). Race teams embracing low pressure vacuum car-burizing have grown steadily. Highly loaded gear applications such as those in 3,500 hp top fuel dragsters (Figs. 7 and 8) require crankshaft fracture toughness values 2 or 3 times normal values. When the bottom end fails in one of these races, the engines literally explode.

Where the Market is Headed
Gear manufacturing costs in % The debate about whether low pressure vacuum carburizing technology is superior to atmosphere car burizing and whether parts traditionally oil quenched can be replaced, in the majority of cases, by high pressure gas quenching techniques has been answered in the affirmative. Demand for vacuum carburizing systems equipped with high pressure gas quenching, oil quenching, or both, has quadrupled since 2000. Commercial heat treating has seen the second largest increase in the number of installed low pressure vacuum carburizing units as shown in Fig. 9. The breakdown of the various industrial segments is: automotive (73.6%), commercial heat treating (15.6%), industrial products (6.9%), and aerospace (3.9%).

Reference:
F.J. Otto and D.H. Herring, Vacuum Carburizing of Aerospace and Automotive Materials, Heat Treating Progress, Jan./Feb. 2005.

For more information:
Frederick Otto is president, Midwest Thermal-Vac Inc., 5727 95th Ave., Kenosha, WI 54409; tel: 262-605-4848; fax: 262-605-4806; e-mail: fredotto@mtvac.com; Internet: www.mtvac.com

Daniel Herring, The Heat Treat Doctor, is president, The Herring Group Inc., PO Box 884, Elmhurst, IL 60126; tel: 630-834-3017; fax: 630-834-3117; e-mail: dherring@heat-treat-doctor.com; Internet: www.heat-treat-doctor.com

Vacuum carburized crankshaft
Fig. 8 — Vacuum carburized AF1410 crankshaft.

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