Physical Properties of Manifold Materials
Abstracted from a Panel Presentation 
at the 107th Ductile Iron Society Meeting, October, 1997

Four participants from an SAE Committee on engine manifold materials reviewed the past, present and future in design, performance, and material properties employed for exhaust manifolds. Silicon-Moly cast ductile irons are emerging as the material of choice and the replacement material for others in order to meet the increasing operating temperature requirements for manifolds.

Design Considerations
Chandran Santanam, Development Engineer
for Exhaust Manifolds, GM Powertrain

SAE Committee Activities
The committee consisting of manifold users, producing foundries, and research institutions, is focusing on helping end users by a four-pronged program: develop design information; produce a material selection guide; produce a standard for material selection and evaluation; and publish technical papers. This work is driven by emerging requirements: service temperatures reaching 1500-1600^oF, extreme temperature differentials, manifolds cannot be water cooled, yield strength is lost in service, premature cracking develops, and industry is requiring 100,000 mile vehicle in-service life. A future goal is 200,000 miles.

Design Procedure
Design must take into account functionality, part/system performance, and exhaust emission characteristics. The manifold design procedures and factors affecting each are:

• Begin with a CAD part design for smooth gas flow.

• Conduct computer simulation gas flow tests.

• Generate FEM for in-service heat flow, heat transfer and imposed strains.

• Dynamometer test on the engine for fatigue resistance (usually 500-1000 hours).

• Dynamometer test for durability (100-200,000 miles).

• Vehicle service test for high engine load and induced peak stresses.

Factors affecting design:

• Packaging – must fit available envelope.

• Weight – reduced for high fuel economy (thin wall design).

• Emissions – HC, C0₂, NO_x emissions lowest feasible.

• Back pressure – handle pressure from exhaust system.

• Sensor adaptability – must accommodate 0₂, EGR, gas temperature sensors, all required for emission control.

• Air gap installation – to protect other engine areas.

• Heat shielding – to protect electronics.

• Gas temperatures – 1800^oF plus anticipated.

• Material capabilities – cast Si-Mo ductile iron, high moly ductile irons and cast stainless have best potential.    back to top

Emission Factors
Trends in requirements are reduced vehicle emissions all around. This requires lean burning engines, high power density, and higher exhaust temperatures. Use of catalytic converters increases temperatures, requires exhaust gas recirculation, requires sensors be installed in the manifold, which results in large, cold operating areas in the manifold, gas flow interruption, and huge operating temperature swings and gradients. Manifold air injections are required for catalytic converter hydrocarbon combustion. On the other hand, to reduce initial engine emissions, and provide maximum fuel economy, higher air/fuel ratios are employed. This complicates temperature distribution in a manifold. The results, as demonstrated by heat transfer computer models, are huge swings and differentials in manifold temperatures in different locations in the same part. Additionally, fastener mounting on the engine, mechanically constrains growth of the manifold in service. The manifold must resist thermal fatigue, hot cracking, differential expansion stressing, and possess high yield strength at high temperatures.

Material Requirements
Cast stainless steels and silico-molybdenum ductile irons offer the best potential to meet the following requirements: corrosion resistance, low thermal conductivity, low thermal expansion coefficients, high temperature fatigue strength, high temperature creep resistance, antioxidation properties and high temperature yield strength.

Material Applications
The initial choice for manifolds was gray cast iron. Exhaust temperatures seldom exceeded 1200oF. By the 1970’s, emission systems forced temperature requirements above 1400^oF, resulting in the introduction of ferritic ductile irons for durability. The 80’s saw increased usage of welded tubular assemblies. But because of cost, noise problems, and durability, cast silicon-molybdenum irons (Si-Mo irons) were introduced. By 1990, exhaust temperatures increased to 1800^oF and beyond, resulting in a switch to cast NiResist and Si-Mo ductile irons. For temperatures exceeding 1800^oF, cast stainless steels are being introduced in some heavy-duty truck applications. However, today Si-Mo irons are the materials of choice, amounting to 58-1/2% of use by the big three auto companies. Ductile irons are still used in about 32% of the models. Tubular constructions are still used in 7-1/2% of heavy-duty applications. 

Welded tubular manifolds are difficult to manufacture, often provide poor fit-up (causing air-in leaks) and cause obstructed, turbulent exhaust gas flows, hence, a choice of cast manifolds. Coupling material and design are everything. A series of manifold applications were illustrated, showing the required design complexibility and the extreme operating temperature differentials. For example, mounting flanges may operate at only 350-400^oF, whereas 2" away ports are 1600^oF or higher, exceeding the yield strength of a material.     back to top

Basic Cast Manifolds
There are four basic types of cast manifolds of interest today shown in the table below.

 For Si-Mo irons, the higher the moly content, the lower the ductility and machinability, but the higher the yield strength and hardness. These irons are usually 10-15% pearlitic and contain intercellular molybdenum carbides. Figures 1-3 illustrate the mechanical properties of Si-Mo and ferritic ductile irons.

Figure 1. Typical Tensile Strengths

Figure 2. Typical Yield Strengths

Figure 3. Typical Elongations

Numerous solidification modeling studies using Magmasoft^® illustrated how large differential thicknesses in various manifold designs present challenges in casting feeding design, and problems in avoiding mold shift for wall thickness down to 3mm of less than 0.8mm. A new challenge facing industry is to develop weldability of ferritic and Si-Mo iron designs by matching chemistry and process techniques.

Material Properties
Richard Gundlach, President,
Climax Research Services

Material Properties

Si-Mo ductile iron, as a preferred choice, offers the following features:

• Inexpensive in composition and producibility.

• Forms a stable ferritic microstructure.

• Has improved oxidation resistance (no oxidation scale flaking if 3.5% Si or higher).

• High upper critical transformation temperature (4% carbon, critical temperature is 1500-1600^oF, and hence will not transform in service).

• Improved elevated temperature properties (Mo increases elevated temperature strength - Si improves room temperature strength raising the minimum 40 to 65 ksi).

• Mo content raises in-service creep strength.

• Si-Mo irons have desirable, reasonably low thermal conductivity and expansion properties.

• Ductility can reach 20%, and Young modulus approaches that of steel.

The primary mode of failure in service is by fatigue. In an operating temperature range of room to 1500^oF, a ferritic casting will expand 1%, and can produce tensile stresses of up to 200 ksi upon cooling down. This continuous cyclic straining and high temperature creep yielding induces a time-delayed, progressive failure - that’s thermal fatigue. As the silicon content increases to 3% plus, room temperature yield strengths increase from 40-65 ksi. Increasing moly content up to 0.8% has a similar percentage increase in the elevated temperature properties.

Thermal Fatigue Properties

Climax Research Services evaluated the fatigue properties for a series of different composition irons. Tests are conducted on machined tensile type bars in uniaxial fatigue straining. Bars are fixtured at each end to prevent being axially pulled apart and cyclically heated by an induction coil between 200^oC and 800^oC. An attached strain indicator measures induced strains (tensile and compressive). Yielding is detected by bar bulging. Stress levels to failure are calculated from the strain data, and plotting the number of thermal cycles to failure. Figure 4 illustrates the maximum service temperatures for various manifold materials. A typical stress level at room temperature for a room to 800C cycle is 35 ksi. This test procedure permits determination of the flow stress during both the manifolds heating and cooling cycles, which can now be used in simulation studies and manifold design considerations. The amount of bulging in fatigue test specimens is an indication of the amount of yielding and creep. The higher the moly content, the less the amount of bulging.

Gray iron manifolds Compacted Graphite iron Ductile iron (d4512)(3) Hi silicon ductile iron (4) Hi silicon and molybdenum (5) Niresist (6) Stainless steel fabricated (7) Stainless steel castings (8)
Figure 4. Allowable manifold temperatures

Fatigue failures initiate at nodule sites and voids. High temperature creep and yielding produces internal voids. These voids are then the initiation sites for failure, due to a "notching effect" or due to internal oxidation around the void. Other failure origins can be areas at DI cell boundaries. These can transform to pearlite on thermal cycling above the critical temperature. Thus, high critical temperatures promoted by Si and Mo contents have a significant beneficial effect. 

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