CF8C-Plus: A New Cast Stainless

Steel for High-Temperature Diesel

Exhaust Components

P.J. Maziasz, and J.P Shingledecker

Oak Ridge National Laboratory, and

M.J. Pollard

Caterpillar Technical Center

Research sponsored by the Heavy Vehicle Propulsion

Materials Program, DOE Office of FreedomCAR and

Vehicular Technology Program, under contract

DE-AC05-00R22725 with UT-Battelle, LLC

CF8C-Plus cast stainless steel was developed to provide higher temperature capability and reliability for

advanced diesel engine

• Cast stainless upgradefor SiMo cast-iron diesel engine exhaust components

turbo-housing

exhaust manifold

C-15, 14.6L HD On-Highway Diesel Engine

Materials Need: High Performance Low-Cost Alloy was Needed to Replace SiMo Cast Iron

Some Candidate Alloy Compositions (wt%)Some Candidate Alloy Compositions (wt%) • SiMo Cast Iron: Fe-3.45C-4Si-0.6Mo-0.3Mn • CF8C: Fe-19Cr-10Ni-0.07C-1.0Nb-0.7Mn-1Si • CF8C-Plus: Fe-19Cr-12Ni-0.07C-0.07Nb-0.4Si-+Mn+N • Ni-Resist: Fe-2Cr-35Ni-0.5Mn-5Si-1.9C

Improving the properties of less expensive alloys without the costly addition of Ni offered the best opportunity

CF8C-Plus = Best Results

Alloy Development: Mn and N were the “Plus” added to improve austenite stability

• Lower cost Mn and N were added instead of costly Ni for fully austenitic stainless steel

• CF8C = 15-25% Delta Ferrite, CF8C-Plus = 0% Delta Ferrite

CF8C-Plus Cast Stainless Steel won a 2003 R&D100 Award for Outstanding Heat-Resistance at 850oC, and

Successful Commercial Scale-Up in only 1.5 years

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18,000 20,000 22,000 24,000 26,000 28,000

CF8C CF8C-Plus SiMo Cast Iron Ni-Resist

STR

ESS

(MPa

)

LMP (C=20)

CF8C CF8C-Plus

SiMo Cast Iron

Ni-Resist

LMP (Larson-Miller Parameter) is calculated using creep-rupture time and temperature

Creep-Rupture at 550-850oC

Engineered Microstructure • CF8C-Plus Has “Super” Creep Resistance at 850oC

Because Abundant, Stable Nano-NbC Precipitates Pin Dislocations

(TEM, as cast)

500 nm

Creep Tested 850°C/500 h

NbC

CF8C-Plus

500 nm

Creep Tested 850°C/23,000 h

NbC

Std CF8C

CF8C-Plus Has Great Castability for Defect-Free Parts

• CF8C-Plus (<0.5Si) shows as-good or better fluidity compared to CF8M (1.5Si) at equivalent pour temperatures

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Leng

th, i

n.

2500 2600 2700 2800 2900 3000 Pour Temperature, F

CF8C Plus #1 CF8C Plus #2 CF8M

Fluidity

From Ron Bird, Stainless Foundry and Engineering

CF8C-Plus Fluidity Spiral:

•Round-top trapezoid, 3/8” w (bottom), ¼” w (top), ¼” high •Length = linear length of spiral

Better

In May, 2007, CF8C-Plus was submitted to ASTM for approval of a new heat-

resistant cast alloy grade – HG10MNN

Welds of CF8C-Plus passed U-bendDuctility test (SF&E)

Welds of CF8C-Plus passed RT tensiletests for UTS and ductility, with 20%Better YS than base metal

Commercial Applications – Direct Replacement of NiResist for Natural Gas Reciprocating Engines at Reduced

Cost (Cost of CF8C-Plus = 80% of NiResist)

45 lb static sand-cast CF8C-Plus exhaust component castby Stainless Foundry and Engineering, Inc.

Caterpillar is now using CF8C-Plus steel for the CRS components which are on all heavy-duty

highway truck diesel engines in 2007

CF8C-Plus steel

SiMo Cast-iron

• Exhaust combustor (turbo exhaust + injected fuel) to clean out particulate filters: high temperature and rapid cycling conditions

New Work on Step-Castings to get mechanical properties data on thin-sections, to support turbocharger

and manifold applications

ORNL is using step-castings of CF8C-Plus and CF8C-Plus Cu/W to measure mechanical properties of the thin sections

representative of heavy-diesel exhaust components

• Stainless Foundry & Engineering made step castings of CF8C-Plus and CF8C-Plus Cu/W in late 2006

Thin Sections of CF8C-Plus have refined dendrite/grain structure

A C E

CF8C-Plus thin sections have YS as good or better than thicker sections

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CF8C: YS CF8C: UTS CF8C-Plus: YS CF8C-Plus: UTS

CF8C-Plus Step Cast: YSCF8C-Plus Step Cast: UTSCF8C-Plus+Cu/W Step Cast: YSCF8C-Plus+Cu/W Step Cast: UTS

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a)

TEMPERATURE (oC)

TEMPERATURE (oF)

STR

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(ksi

)

YS CF8C-Plus Cu/W

Preliminary data indicates CF8C-Plus thin-sections also have good creep and rupture resistance

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EEP

STR

AIN

(%)

TIME (hrs)

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0.5%

CF8C-Plus Cu/W has improved creep resistance compared to CF8C-Plus

• Good creep resistance in thin-section casting is important for turbochargerand manifold applications

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CF8C-Plus CF8C-Plus+Cu/W

CR

EEP

STR

AIN

(%)

TIME (hrs)

Step Castings (1/2" Thick) Creep Test: 800 oC 75MPa

Conclusions for New CF8C-Plus cast austenitic stainless steel

• Castable, even as thin sheets • Weldable • Outstanding Creep Performance • Cost-Effective • Applications

– Caterpillar CRS components (on-highway in 2007) – Exhaust components for NG engines – Turbocharger housings – Numerous other potential spin-offs