Steel products may be manufactured either by casting or forging steel. Steel casting is the process by which a metal is heated until it reaches a liquid state and then poured into a mold that shapes the desired product. Steel forging implies the application of mechanical forces to heated solid blocks of steel (such as ingots and/or billets) that are shaped into desired products permanently.
Both manufacturing processes require the application of high temperatures to steel raw materials (to liquify or make it malleable) and the execution of CNC machining work at the end of the process to obtain the final product. Final products may also undergo surface finish treatment, such as painting, powder coating, polishing, various types of coating (for example zinc plating) and wear protection/hardening (application of tungsten carbide overlay).
Last but not least, cast and forged parts may be assembled, welded, brazed, hard-faced before being shipped as final products.
The products resulting from casting and forging processes have different properties in terms of surface porosity (generally better for forged vs. cast products), grain structure (finer for forged products), tensile strength (generally superior for forged products) and fatigue resistance. These alternative manufacturing processes are therefore used (and suited for) different circumstances and applications.
The casting process is preferred for:
- parts and components that would be too complex or expensive to manufacture by steel forging (example: large valve bodies);
- parts that have internal cavities;
- large sized parts (there are virtually no size limits in terms of the weight of the parts that can be produced with the casting process);
- parts in special alloys (some specific alloys are more difficult to forge than cast, for example, those with a high content of Nickel and Moly, which have considerable resistance to mechanical forces);
- parts requiring mass production and small lots.
The forging process is preferred for:
- parts requiring extremely high strength, toughness, and resistance (indeed, during the forging process the steel grain structure gets modified to conform to the shape of the final product – with high uniformity of composition and metallurgical recrystallization);
- parts that have to withstand stronger impacts and mechanical forces;
- parts where porosity, the risk of a gas void, pockets and the possible formation of cavities (even micro-granular) are not acceptable;
- production of mechanically strong parts without using expensive alloys;
- parts that require high wear resistance;
- parts subject to high loads and stress;
- high-end applications when the integrity and the quality of the part is the main objective in the production process, rather than time and cost.
The evolution of the casting technologies has reduced the gap between the physical properties of cast vs. forged products making modern cast products very competitive in terms of quality, strength, and wear resistance: however, in many fields, steel forging remains, still, the preferred manufacturing option (example: small sized valves, i.e. forged valves, or high-pressure valves).
Read about forging steel on Wikipedia.
The main types of casting processes are:
Sand casting is the most traditional casting method and consists in pouring liquid metal into binders that resist the molten metal (such as clay bonded/green sand hard bonded/resin, thermosetting resin sand, and shell).
This term refers to precision molding executed by injecting the liquid metal into a metal die and a ceramic coating. The mold material can be hard wax, lost wax, lost foam and similar.
These processes are used for different applications in terms of parts dimension (sand casting is used for large parts, investment casting for small parts up to 100 kilograms and 1,5 meters of max. length), allowed tolerances (investment casting create more precise parts) and cost targets (investment castings tend to be more economical than sand casting).
ORIGIN OF STEEL FORGE
Steel forging appeared in China in the ancient ages to produce various types of metal products.
Whereas the procedures and the tools used to produce forged parts have changed through the centuries (from the use of anvils, hammers, and manpower to automated machines as hydraulic presses) the basic steel forging process is still based on the application of thermal energy to solid blocks of steel and their further processing into finished products by the application of mechanical (hammering) forces.
The basic process of forging consists in a few traditional steps:
- the raw material (steel blocks, ingots, billets) is cut into smaller parts if needed
- the raw material is heated to reach the required forging temperature (the application of heat is necessary to make the material ductile and malleable); the forging temperature depends on the type of metal and is achieved by positioning the material into a furnace/oven;
- the heated metal is shaped into the required form by applying mechanical forces (pressure).
- the semifinished part undergoes machining, finishing and heat treatment
At the end of the process, the resulting product features extreme strength, impact toughness and wear resistance thanks to the metallurgical recrystallization and grain refinement resulting from the applied thermal and mechanical treatment.
STEEL FORGING TYPES
Depending on the temperature applied to the raw material during the forging process, forging is classified into:
- Cold forging steel: when no heating is applied, i.e. the forging process happens at room temperature (higher mechanical forces are needed in this case and the metal has lower formability vs. hot or warm forging methods)
- Warm forging steel: the raw material is heated at temperatures between 800 and 950/1000 C°
- Hot forging steel: when the heating temperature is above 950/1000 C° (and generally below 1300 C°) to give the metal high ductility and make the forging possible even with the application of modest mechanical pressures.
TYPES OF STEEL FORGING
The “Closed Die Forging Steel” is a forging process in which the dies move towards each other and covers the workpiece in whole or in part. The heated raw material, which is approximately the shape or size of the final forged part, is placed in the bottom die.
The “Open die forging steel” is the process of deforming a piece of metal between multiple dies that do not completely encapsulate the material. The metal is shaped by the action of the dies that “hammer” or “stamp” the material through a series of movements until the required shape is achieved.
THE DIFFERENCE BETWEEN OPEN-DIE AND CLOSED-DIE FORGING STEEL
FIELDS OF APPLICATION OF STEEL CASTING AND FORGING
Steel casting and forging are used to produce parts for the following industries:
- petrochemical plants (example forged valves, forged fittings, flanges, etc)
- power generation and waste processing
- mining and mineral processing
- agriculture and livestock handling
- water treatment
- automobile industry (pulleys and gear wheels)
- materials handling
- asphalt plants
- stormwater parts
- rendering plants
FORGED VS CAST MATERIALS GRADES FOR VALVES
The chart shows the matching materials grades for cast and forged valves. For instance, ASTM A216 WCB is the closer cast grade material to ASTM A105N forged grade (and so on).
|FORGED VALVE MATERIAL||CAST VALVE MATERIAL|
|ASTM A105N||ASTM A216 WCB/WCC|
|ASTM A350 LF2||ASTM A352 LCB/LCC|
|ASTM A350 LF3||ASTM A352 LC3|
|ASTM A182 F5A/F5||ASTM A217 C5|
|ASTM A182 F9||ASTM A217 C12|
|ASTM A182 F11||ASTM A217 WC6|
|ASTM A182 F22||ASTM A217 WC9|
|ASTM A182 F91||ASTM A217 C12A|
|AUSTENITIC STAINLESS STEEL|
|ASTM A182 F304/F304L||ASTM A351 CF8/CF3|
|ASTM A182 F316/F316L||ASTM A351 CF8M/CF3M|
|ASTM A182 F347||ASTM A351 CF8C|
|ASTM A182 F321|
|ASTM A182 F44 (6MO)||ASTM CK3MCuN|
|ASTM A182 F20 (ALLOY 20)||ASTM A351 CN7M|
|FERRITIC-AUSTENITIC STAINLESS STEEL|
|ASTM A182 F51 – UNS S31803 (DUPLEX S.S.)||ASTM A351 CD3MN|
|ASTM A182 F53 – UNS S32750 (SUPER DUPLEX S.S.)||A890 GR.5A CE3MN*|
|ASTM A182 F55 – UNS S32760 (SUPER DUPLEX S.S.)||A890 GR.6A CD3MWCuN|
|INCONEL 825 – UNS N08825 ASTM B564-N08825||A484 CU 5MCuC*|
|INCONEL 600 – UNS N06600 ASTM B564-N06600||A494 CY40*|
|INCONEL 625 – UNS N06625 ASTM B564-N06625||A494 CW6MC*|
|MONEL 400 – UNS N04400 ASTM B564-N04400||A494 M35-1|
|ASTM B381 GR.F2||ASTM B367 GR.C2|
|ASTM B381 GR.F3||ASTM B367 GR.C3|