Steel products can be made by either casting or forging steel. Steel casting is the process of heating a metal until it turns into liquid form and then pouring it into a mold that shapes the intended product. Steel forging is the permanent shaping of heated solid blocks of steel (such as ingots and/or billets) into desired products using mechanical forces.
Forged steel vs. Cast steel
To obtain the final result, both techniques of production necessitate the application of high temperatures to steel raw materials (to liquify or make them pliable) and the execution of CNC machining work at the end of the process. Surface finish treatment of final products may include painting, powder coating, polishing, different types of protective layers (for example, zinc plating), and wear protection/hardening (application of tungsten carbide overlay).
Finally, cast and forged components can be assembled, welded, brazed, and hard-faced before being dispatched as finished products. Surface porosity (which is usually better in forged items than in castings), grain structure (which is finer in forged items), tensile strength (which is usually higher in forged components), and fatigue resistance vary between casting and forging procedures.
As a result, these alternative manufacturing techniques are utilized (and adapted for) a wide range of conditions and applications.
The casting technique is appropriate for:
- Parts and components that would be too complex or costly to manufacture by steel forging (for example, massive valve bodies)
- Parts with internal cavities
- large parts (there are no size restrictions in terms of the weight of the parts that can be manufactured using the casting method)
- Parts in special alloys (certain specialized alloys, such as those with a high percentage of nickel and molybdenum, which have high mechanical resistance, are more difficult to forge than cast)
- Parts require mass production and small lots.
Forging is the method of choice for:
- Parts needing significantly higher strength, toughness, and resistance (in reality, the steel grain structure is altered during the forging process to fit the final product’s shape with high composition uniformity and metallurgical recrystallization)
- Components that must sustain higher impacts and mechanical forces
- Components where porosity, the possibility of a gas void, and the formation of cavities (even micro-granular) are prohibited
- Production of mechanically robust parts without the use of expensive alloys
- Parts with a high wear resistance requirement
- Parts that are subjected to high loads and stress
- High-end applications where the integrity and quality of the part are the primary targets in the manufacturing process, rather than time and cost.
The advancement of casting technology has narrowed the gap between the physical attributes of the cast and forged products, despite being competitive in terms of quality, strength, and wear resistance, steel forging is still the preferred manufacturing technique in many industries (example: small-sized valves, i.e. forged valves or high-pressure valves).
Casting of Steel
The main types of casting processes are:
- Sand Casting
The most conventional casting method is sand casting, which involves pouring liquid metal into binders that resist the molten metal (such as clay bonded/green sand hard bonded/resin, thermosetting resin sand, and shell). It is typically used for bigger components.
- Investment Casting
The mold material can be hard wax, lost wax, lost foam, or anything similar. The liquid steel is injected into the ceramic coating or metal dye in the process of investment casting. It is typically utilized for smaller, more delicate components.
In terms of component size (sand casting is used for large parts, investment casting for small parts up to 100 kg and 1.5 meters in maximum length), allowed tolerances (investment casting produces more accurate parts), and financial targets, these processes are used for a variety of reasons (investment castings tend to be cheaper than sand casting).
Forging of Steel
Although the procedures and tools used to produce forged parts have evolved over the time (from anvils, hammers, and manpower to automated machines such as hydraulic presses), the basic steel forging process remains focused on the application of thermal energy to solid blocks of steel and their subsequent processing into finished products by the application of mechanical (hammering) forces.
Steps Involved In The Process Of Forging
The fundamental forging process consists of a few basic steps:
- If necessary, the raw material (steel blocks, ingots, or billets) is cut into smaller parts.
- The raw material is heated to the required forging temperature (heating is required to make the material ductile and malleable); the forging temperature varies by metal type and is achieved by positioning the material in a furnace or oven;
- The heated metal is shaped into the required shape by applying mechanical forces (pressure).
- The semi-finished part is machined, finalized, and heat treated.
The metallurgical recrystallization and grain refinement caused by the applied heat and mechanical treatment result in a product with high strength, impact toughness, and wear resistance at the end of the process.
Forging is divided into 3 types based on the temperature applied to the raw material during the forging process:
- Cold Forging Steel: The cold forging process takes place at room temperature when no heating is involved (higher mechanical forces are needed in this case and the metal has lower formability as compared to hot or warm forging methods).
- Warm Forging Steel: the raw material is heated to temperatures ranging between 800 and 950/1000 degrees Celsius.
- Hot Forging Steel: when the heating temperature is greater than 950/1000°C (and usually less than 1300°C) it gives the metal high ductility and allows forging at low mechanical pressures.
Open and Closed Die Forging of Steel
- Closed-Die Forging
“Closed Die Forging Steel” is a forging method in which the dies move towards one another and cover the workpiece completely or partially. The heated raw material, which is roughly the shape or size of the finished forged object, is placed in the bottom die.
- Open-Die Forging
The technique of deforming a piece of metal between many dies that do not entirely encapsulate the material is known as “open die forging steel.” The metal is shaped by the action of dies, which “hammer” or “stamp” the material in a sequence of movements until the desired shape is obtained.
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