Overview to ADI….

ADI was discovered almost 50 years ago and found successful commercial application in 1972. Today ADI has become the material of choice for the designer, as it offers the best design combinations of low cost, design flexibility, machinability, high strength to weight ratio, good toughness, wear resistance & fatigue strength. ADI is more environment friendly than many competing materials.

Overview to ADI….

Material selection, processing, cost, product design, ease of availability, environmental impact due to its use, & performance of the final product are inseparable. ADI comes out with high scores on all these counts.

Making of ADI…

Molten Cast iron when treated with magnesium results in nodular iron on freezing.

Graphite is present in a spheroidal form rather than as flakes.

Ductile iron has superior mechanical properties as compared to cast iron & is cheaper than malleable iron.

Malleable irons were manufactured by subjecting white cast irons to very long heat treatment cycles, and hence were very expensive.

Making of ADI…

Ductile iron has replaced malleable irons entirely & steel / cast iron in many applications.

Ductile iron when heated to the austenitic range [900o C approx] and quenched in the range of 280o C to 500o C gives us Austemperted Ductile Iron [ADI].

The original inventors called the process “incomplete austempering” as the resultant microstructure contained large quantities of RETAINED AUSTENITE [RA]. Incomplete austempered ductile iron exhibited high elongation values as compared to normalized, hardened or fully bainitic ductile irons for similar tensile strength levels.

Advantages 0f ADI…Advantages 0f ADI…1. Lower in cost when compared to other competing

manufacturing methods, such as fabrication, wrought /machined, forged / machined.

2. Superior mechanical properties as compared to cast iron, weldments, aluminum alloys, normalized or toughened ductile irons.

3. Energy consumption for a finished ADI product is almost half that of a forged, machined & case carburised hardened component, for a similar application.

4. Ductile irons will exhibit superior damping capacity & machinability as compared to steel.

Advantages 0f ADI…Advantages 0f ADI…

5. ADI has a very low cost to strength ratio when compared to other commonly used engineering materials.

Material. Forged Al.

Cast Al.

Cast Steel

Forged Steel

H&T Steel

Ductile Iron

ADI.

Unit Cost / Unit Y.S.

20 12 6 4 3 2 1

Advantages 0f ADI…Advantages 0f ADI…

6. ADI exhibits transformation induced plasticity [TRIP]. Hence it has superior wear resistance as compared to irons & steels of equivalent hardness values. An ADI with a hardness value of 350 BHN (38 HRC) will have a wear resistance equivalent to a case hardened steel of 60 HRC, making ADI a suitable choice for gears, camshafts, rollers, & other wear parts. Wear resistance of ADI is further enhanced by shot peening.

7. Due to TRIP, ADI also has higher toughness & fatigue strength as compared to normalized / toughened ductile iron, nearing those of steel at times, especially for the lower tensile grades.

Prior Metallurgical Parameters to Prior Metallurgical Parameters to Ensure Success with ADI…Ensure Success with ADI…

1.1. Good foundry practice to ensure a sound casting Good foundry practice to ensure a sound casting free from shrinkage & porosity. free from shrinkage & porosity. The austempering The austempering process cannot compensate for foundry process cannot compensate for foundry defectsdefects

2.2. Maintenance of proper and consistent chemistry. Maintenance of proper and consistent chemistry. Recommended limits for chemical composition are Recommended limits for chemical composition are as follows: as follows: Carbon: 3.5/3.9%; Silicon: 2.3/2.7%; Carbon: 3.5/3.9%; Silicon: 2.3/2.7%; Manganese: 0.25% max; Copper 0.8% max; Manganese: 0.25% max; Copper 0.8% max; Nickel:2.0% max; Molybdenum: 0.25% max Nickel:2.0% max; Molybdenum: 0.25% max only for heavy sectioned castings. only for heavy sectioned castings.

3.3. Nodularity should be more than 80% & nodule Nodularity should be more than 80% & nodule count should be more than 100/mm2count should be more than 100/mm2

4.4. Minimum 50% pearlite in the matrix & less Minimum 50% pearlite in the matrix & less than 1% carbides prior to austempering.than 1% carbides prior to austempering.

Nodularity…Nodularity…

Microstructure of ductile irons of varying degrees of nodularity. (a) 99% nodularity. (b) 80% nodularity. (c) 50% nodularity. All unetched. 36×

Nodule Count…Nodule Count…

Series of micrographs depicting increasing nodularity in ductile irons. Upper left: 50 nodules per 10 mm square. Upper right: 100 nodules per 10 mm square. Lower left: 150 nodules per 10 mm square. Lower right: 200 nodules per 10 mm square. As-polished. 100×

Heat Treatment of ADI…Heat Treatment of ADI…

AUSTENITISATION is typically done in the temperature range AUSTENITISATION is typically done in the temperature range of 830 to 950of 830 to 950oo C. Care should be taken to prevent decarburization, C. Care should be taken to prevent decarburization, carbon pick up & thermal shock during austenitisation. If salt carbon pick up & thermal shock during austenitisation. If salt baths are used for heating, they should be free of cyanide as carry baths are used for heating, they should be free of cyanide as carry over of cyan salts to the NITRITE/NITRATE BATH will lead to over of cyan salts to the NITRITE/NITRATE BATH will lead to splashing of hot salt & can cause severe injury. QUENCHING is splashing of hot salt & can cause severe injury. QUENCHING is normally done in a NITRITE / NITRATE bath with or without normally done in a NITRITE / NITRATE bath with or without water additions. Whenever water additions are done, proper water additions. Whenever water additions are done, proper stirring is essential as the steam generated can result in splashing. stirring is essential as the steam generated can result in splashing. Time & Temperature for transformation are decided on the Time & Temperature for transformation are decided on the following basis:following basis:

Heat Treatment of ADI…Heat Treatment of ADI…

Selection of time & temperature are decided on the basis of:Selection of time & temperature are decided on the basis of:1.1. Section thickness of the componentSection thickness of the component2.2. Grade of ADI: Higher the tensile, lower will be the Grade of ADI: Higher the tensile, lower will be the

quenching temperature, but never below the Ms quenching temperature, but never below the Ms Temperature. Quenching temperatures are normally between Temperature. Quenching temperatures are normally between 250 to 425o C. Quenching time varies between 20 to 150 250 to 425o C. Quenching time varies between 20 to 150 minutes. Normally higher the tensile strength required, minutes. Normally higher the tensile strength required, higher will be the temperature.higher will be the temperature.

3.3. Pearlite content in the matrix prior to austenitisation.Pearlite content in the matrix prior to austenitisation.4.4. Higher the austenitisation temperature greater will be the Higher the austenitisation temperature greater will be the

RA, greater will be the austenite carbon content & the ferrite RA, greater will be the austenite carbon content & the ferrite will be coarser. The austenitisation conditions should be will be coarser. The austenitisation conditions should be selected, with an aim for optimizing the mechanical selected, with an aim for optimizing the mechanical properties. properties.

Response to Heat Treatment…Response to Heat Treatment…

Unlike for steels, transformation curves for ductile iron grades Unlike for steels, transformation curves for ductile iron grades are not available easily. Hence one has to use personal are not available easily. Hence one has to use personal experience & the response of specific grades. The guiding experience & the response of specific grades. The guiding principle should be achievement of an ausferrite structure & principle should be achievement of an ausferrite structure & correct hardness. Normally one has to establish process correct hardness. Normally one has to establish process parameters, using the limits of tolerance for martensite & parameters, using the limits of tolerance for martensite & bainite in the final structure and the hardness specified. bainite in the final structure and the hardness specified.

Ausferrite….(2% Picral)Ausferrite….(2% Picral)

Ausferrite…(2% Nital)Ausferrite…(2% Nital)

Strain Hardening….Strain Hardening….

Bad Nodularity….Bad Nodularity….

ADI – Showing Carbides due to ADI – Showing Carbides due to presence of W 0.2%...presence of W 0.2%...

1. Always look out for an AUSFERRITE structure & correct hardness after austempering. Upto 3% martensite & 10 % bainite may be present with the rest being AUSFERRITE. The 750 N/mm2 grade may also have some ferrite.

2. Don’t look for BAINITE just because austempering has been done. A bainitic structure of the same hardness will give lower elongation values than an AUSFERRITE structure

3. Tempering or reheating ADI leads to a loss of toughness. Hence please avoid this practice.

ADI is used in diverse components operating under extreme engineering conditions. ADI is used for manufacturing crankshafts, camshafts, gears, sprockets, wear parts in earthmovers, breaker body, suspension/chassis members etc. With the above mentioned advantages many more components are in the development / change over phase to be converted to ADI. Many organizations are studying the benefits of ADI so as to reduce weight & cost of their finished products. ADI offers a great opportunity for value addition & cost reduction to the engineering industry. ADI has very wide spread applications and will definitely be a major growth area in the days to come.