Austempered Ductile Iron
Metals play a significant role in human life since the Bronze Age. Metals’ important advantages include higher toughness and predictable fracture behavior in all directions, which are fundamentally essential for engineering applications. Machining has been a prominent part of manufacturing till date. According to available research data on machining, more than 70% of the products manufactured are machined. One of the most important metals for engineering applications and machining is cast iron. The term cast iron refers to an alloy of iron containing more than 2.0 percentage of carbon. It is usually considered as being brittle. Cast iron is produced by treating the molten pig iron with Magnesium, Cerium, or their combination. The addition of these element(s) cause spheroidal graphite to grow during solidification. This cast iron is called S.G. iron, nodular iron or ductile cast iron. Ductile iron is easy to cast, has good tensile strength, good ductility and is easy to machine. It offers the designer a unique combination of mechanical properties. The matrix may vary from a soft ductile ferritic structure through a higher strength pearlitic structure to a hard and comparatively tough martensitic structure. Heat treatment is a valuable and versatile tool for extending both the consistency and range of properties of Ductile Iron castings beyond the limits of those produced in the as-cast condition. Thus, to fully utilize the potential of cast iron, foundries usually give it some further treatment. The most important heat treatments and their purposes are: stress relieving, annealing, normalizing, hardening and tempering, surface hardening and austempering.
Austempering of ductile cast iron improves its toughness, particularly in that hardness range in which conventionally hardened and tempered steels are susceptible to a reduction in their impact strength. Austempering therefore is better in some cases where conventional hardening and tempering is not useful. Apart from a slight distortion, austempering of ductile iron does not produce excessively large stresses.
The Austempering Process
Austermpring changes the microstructure of the ductile cast iron. The reaction can be divided in two stages:
The austempering period and temperature therefore play a crucial role in the mechanical properties of the ductile cast iron. The austempering temperature is selected to give the required grade of alloyed ductile iron.
Austempering is comparatively a high cost treatment and is used in high precision machined components where the risk of cracking has to be kept to a minimum. The austermpering of machined components especially is critical and requires a thorough study as the process induces a very slight but unavoidable distortion. Manufacturers in India can afford to austemper cast iron as labour is comparatively inexpensive.
Austempered ductile irons are the most recently developed materials of the ductile iron family. Austempering results in metallurgical structures that provide properties favourably comparable to those of steel while taking advantage of a near-net-shape manufacturing process.
Austempered ductile iron market in India and abroad has begun to rapidly increase from a modest beginning in the early 1970’s. The use of austempering heat treatment and its effects to produce a material with desirable properties were confined to research experiments, till the first commercial production of a small crankshaft for a hermetically sealed refrigerator compressor was successful. Because it is tougher than ductile cast iron, austempered ductile iron is used for applications that need toughness – for example for parts like gears, shafts, differentials, etc. Typically, austempered iron is used as a replacement for steel because of its high tensile strength and low cost. It is widely used today in the automobile, tractor and mechanical industries owing to its excellent damping capacity, high wear resistance and low fabrication cost. Stringent environment norms with respect to CO2 emissions are encouraging automobile manufacturers to shift to higher strength to weight materials in their engines.
Austempered Ductile Iron Standards
Austempering is an isothermal heat treatment process of cast iron and ferrous materials that enrich its mechanical properties. Isothermal processes are processes carried out at the same temperature.While ductile cast iron (aka spheroidal graphite iron or nodular iron) manufacturing was developed in the late 1940's, the austempering process itself was developed in the 1930s. It’s commercial use however, started in the late 1970’s.
Due to its inherent mechanical properties, ductile cast iron has varied uses as machined components. The austemperised ductile iron has a strength-to-weight ratio that exceeds that of aluminium. Ductile iron castings are easily machinable before heat treatment, and their casting properties (viz high strength, toughness, wear resistance and ductility) are enhanced after machining through the austempering process. The demand for austemperised ductile cast iron is ever increasing not only in India but from Europe and other parts of the world as well.
Austempering of ductile cast iron improves its toughness, particularly in that hardness range in which conventionally hardened and tempered steels are susceptible to a reduction in their impact strength. Austempering therefore is better in some cases where conventional hardening and tempering is not useful. Apart from a slight distortion, austempering of ductile iron does not produce excessively large stresses.
The Austempering Process
Austermpring changes the microstructure of the ductile cast iron. The reaction can be divided in two stages:
- Austenite decomposes to ausferrite (carbon enriched austenite + bainitic ferrite)
- The carbon enriched austenite decomposes to ferrite and carbide
The austempering period and temperature therefore play a crucial role in the mechanical properties of the ductile cast iron. The austempering temperature is selected to give the required grade of alloyed ductile iron.
Austempering is comparatively a high cost treatment and is used in high precision machined components where the risk of cracking has to be kept to a minimum. The austermpering of machined components especially is critical and requires a thorough study as the process induces a very slight but unavoidable distortion. Manufacturers in India can afford to austemper cast iron as labour is comparatively inexpensive.
Austempered ductile irons are the most recently developed materials of the ductile iron family. Austempering results in metallurgical structures that provide properties favourably comparable to those of steel while taking advantage of a near-net-shape manufacturing process.
Austempered ductile iron market in India and abroad has begun to rapidly increase from a modest beginning in the early 1970’s. The use of austempering heat treatment and its effects to produce a material with desirable properties were confined to research experiments, till the first commercial production of a small crankshaft for a hermetically sealed refrigerator compressor was successful. Because it is tougher than ductile cast iron, austempered ductile iron is used for applications that need toughness – for example for parts like gears, shafts, differentials, etc. Typically, austempered iron is used as a replacement for steel because of its high tensile strength and low cost. It is widely used today in the automobile, tractor and mechanical industries owing to its excellent damping capacity, high wear resistance and low fabrication cost. Stringent environment norms with respect to CO2 emissions are encouraging automobile manufacturers to shift to higher strength to weight materials in their engines.
Austempered Ductile Iron Standards
Each country has different norms and different grades that govern the production of austempered cast iron. Standards in India are different from standards in say China, and foundries need to ensure that they satisfy the norms of the customer’s country of origin. That is why it is necessary to ensure that you outsource your requirements of austempered ductile iron to a reputed foundry that adheres to strict quality control standards.