There currently is a lot of debate over which is best, forging or casting.
Much of the reason for the debate is economic. For centuries ‘forged’ has been specified for tools and hardware that had to be the most durable and of the best quality Manufacturers of less expensive cast goods try to convince their customers that cast is as good as forged.

Pengirim : Winarto

Dear Milist MIGAS

Sekedar sharing pengetahuan:

Benefit of Forging Vs Casting:

There currently is a lot of debate over which is best, forging or casting.
Much of the reason for the debate is economic. For centuries ‘forged’ has been specified for tools and hardware that had to be the most durable and of the best quality Manufacturers of less expensive cast goods try to convince their customers that cast is as good as forged.

The best, perfect, casting may theoreticaly approach a forging in performance but can never achieve the exact same properties. And castings are rarely perfect. Castings often hide hidden defects below the surface (cracks, porosity and sand inclusions) that would be exposed as defects in a forged billet. When a steel billet is created it is rolled or forged, improving its structure. This also closes porosity, welds cracks and there are no sand inclusions from molding. When an item is forged from this billet its integrity is proven again under the hammer and its structure is further refined. (Referrence: http://www.peddinghausanvils.com/forging_benefits.htm)

Difference between Forging and Casting:

1. Process:

Most steel components start as castings: metal that has been melted, poured into a mold and solidified. In the casting process at the foundry, because the mold has the shape of the desired component, all that remains to be done after casting are the various finishing operations.

With forgings, the first shape is an ingot or continuously cast billet. Ingots are large, usually rectangular in form and weigh up to several tons. Ingots or continuously cast billets are forged into shapes by hammers or presses. Extensive machining to final configuration usually is required, and welding also may be necessary before finishing operations can begin.

2. Section Tickness and Shape

In forging, metal is moved while it is still in the solid state. Because the forging billet is solid, substantial force is required to change its shape to the desired configuration. Because of this, the required force increases as section size increases. In practical terms, there is a limit on size and section thicknesses produced by forging.

This doesn’t mean that very heavy sections are never forged. But when they are, relatively little deformation or reduction in cross-section occurs. In other words, the surface of the part merely is moved from one place to another.

In contrast, in the casting process the metal starts as a liquid and flows into the desired shape. Therefore, it is practical to cast components of large sizes and section thicknesses. For extremely large components, cast/weld construction is generally preferable to forged/weld construction. The reason is that fewer
parts are typically involved, and because steel castings tend to have better weldability than steel forgings.

In forging, solid metal is forced into the die cavity. In casting, liquid metal is poured into the mold cavity. Liquids can flow almost anywhere. Therefore, as complexity of shape increases, the practicality of forging decreases. Castings can accommodate great complexity of shape

3. Composition

The question of composition has two parts: what is obtainable from foundries and forging shops, and what is or isn’t castable or forgeable?

Forgings are produced from billets obtained from a steel mill and in compositions produced by the mill. Mills tend to produce limited grades of steel and special orders can be prohibitively expensive. Because steel foundries are more flexible, the number of chemical compositions obtainable from steel foundries is virtually unlimited.

Although a single foundry cannot supply every conceivable alloy, it is always possible to obtain a unique composition to meet a unique requirement from a variety of foundries at lower cost than competitive product forms.

The presence of controlled amounts of ferrite in certain stainless steels leads to increased corrosion resistance, higher crack resistance and better weldability. Ferrite occurs normally in most cast stainless steels, with the ferrite level controllable to produce the desired combination of characteristics. However, ferrite impairs hot working properties and is normally not present in forged components.

The important class of work-hardenable steels also are not forgeable.

Work-hardenable steels are generally high-manganese (approximately 13% Mn) alloys that become harder the more they are worked. Thus, they are ideal for dipper teeth, compactor feet and other earth-moving and excavation applications.

Mechanical Characteristics

The principal mechanical properties of interest to designers are strength, ductility and hardness. But how does the user know the mechanical characteristics of a part?

For cast steel, it is relatively easy. If the component is made from a standard alloy, the characteristics are given in a standard specification. If it is made from any other alloy, standard foundry tests will provide the answers. The values will apply to that component regardless of the axis along which measurements were made.

Many metal parts are made from rolled products like bars or plates. The rolling process changes the properties of the metal. The major advantage is that the strength is increased in the rolling direction or the longitudinal axis. Both forgings and fabrications have directional properties as a result of the rolling process.

With respect to the mechanical characteristics of a forging, most forging references provide only longitudinal characteristics. To obtain the transverse or axial characteristics, the user will probably have to request them specifically. (Refference: http://www.sfsa.org)

How FORGINGS compare to Castings

a.. Forgings are stronger. Casting cannot obtain the strengthening effects of hot and cold working. Forging surpasses casting in predictable strength properties – producing superior strength that is assured, part to part.

b.. Forging refines defects from cast ingots or continuous cast bar. A casting has neither grain flow nor directional strength and the process cannot prevent formation of certain metallurgical defects. Preworking forge stock produces a grain flow oriented in directions requiring maximum strength. Dendritic structures, alloy segregation’s and like imperfections are refined in forging.

c.. Forgings are more reliable, less costly. Casting defects occur in a variety of forms. Because hot working refines grain pattern and imparts high strength, ductility and resistance properties, forged products are more reliable. And they are manufactured without the added costs for tighter process controls and inspection that are required for casting.

d.. Forgings offer better response to heat treatment. Castings require close control of melting and cooling processes because alloy segregation may occur.
This results in non-uniform heat treatment response that can affect straightness of finished parts. Forgings respond more predictably to heat treatment and offer better dimensional stability.

e.. Forgings’ flexible, cost-effective production adapts to demand. Some castings, such as special performance castings, require expensive materials and process controls, and longer lead times. Open-die and ring rolling are examples of forging processes that adapt to various production run lengths and enable shortened lead times. (Refference: https://www.forging.org/facts/faq3.htm)

Advantages of Casting vs Forging : Mainly the complexity of the shape (bentuk hasil tuangan yang rumit/complex seperti blok mesin kendaraan hanya bisa diproses dengan casting dan tidak dengan forging). In casting, liquid metal is poured into the mold cavity. Liquids can flow almost anywhere. Therefore, as complexity of shape increases, the practicality of forging decreases. Castings can accommodate great complexity of shape.