Stainless steel was first recognized as a commercial proposition in 1913, by Harry Brearley, a metallurgist in Sheffield, England, after he noticed that certain gun barrels containing around 13% chromium didn’t rust when they were left outside. What he’d discovered was a steel that approximates to what we know today as type 420 stainless, a grade used to make table knives. Commercial production of this material didn’t start until after the first world war, by which time Doctors Strauss and Maurer at Krupp in Germany were busy discovering what we know as the 18-8 stainless steels. Britain started production of these steels in 1923, under license from the Krupp patent.

This represented the beginnings of stainless steel production and the first steps in a long fight against the many corrosive media which were lurking around to eat up anything that was a piece of steel.

The corrosion resistance of stainless steels is due to the presence of an extremely thin protective oxide film, the so-called “passive film” which forms spontaneously on the steel surface when it’s exposed to air or some other gas or liquid which can supply oxygen to the steel surface. In fact, stainless steel is like humans, in that it needs oxygen to survive. This film is transparent and tightly adherent to the steel, and is so thin, probably less than 0.000001” thick, that it is absolutely invisible while in contact with the surface it’s formed on. The film is insoluble in water and in many other liquids, and impermeable to these and to many gasses.  While it remains intact and tightly adherent to the steel surface, there will be protection from corrosive attack. It seems strange that a huge, heavy weight stainless steel forging owes its corrosion durability to a microscopically thin film, but that’s the way it is. And even if the film is broken locally, serious corrosion will not result providing oxygen is present because the film spontaneously repairs.  If the ruptured film is prevented from repairing itself, corrosion of the steel will continue and will result in either pitting or general attack over the steel’s surface. The protective value of the oxide film on stainless steel will increase with its chromium content. If the steel contains significant amounts of other metals such as nickel or molybdenum, the oxides of these metals will also be present in the film and will improve its resistance to certain types of corrosive attack.

Look for additional blogs on this subject within steelforge.com.  All Metals & Forge Group is a manufacturer of stainless steel forgings and seamless rolled rings for industrial uses, including the PH grades.  This allows AMFG to manufacture the widest range of forged shapes for use in the Industries Served listed on steelforge.com.

We live with metals. Everywhere we turn we see metals. Perhaps we wonder at times where these metals come from. Well, originally, they came out of the ground, on occasion quite deep in the ground, as ores. In other words they were mined. Mining is not a simple process.  It takes large, powerful equipment with boring heads, shovels, blades, or buckets with control arms and shafts made of special alloys, many of which are forged.

Corrosion abrasion, also known as abrasive corrosion, corrosion wear, or slurry erosion, is a significant problem in the mining industry. It is a combined process where both corrosion and abrasion occur simultaneously, often leading to more severe damage to metal parts than either process alone. Thus the need for strong, resistant steels to stand up to these conditions, and for fine-grained forgings, made from closely controlled feedstock, to ensure the required mechanical properties, wear resistance, and corrosion resistance. 

Mining equipment is in constant need of repair, maintenance, replacement parts, or upgrades. Metal surfaces coming in contact with rock or earth include buckets on backhoes, excavators, cable shovels, continuous miners, dippers, and bulldozer blades.  Other equipment includes draglines, dredging machines, hard rock mining machines, drillers, earthmovers, and loaders.  Depending on the mining operation, equipment may include tunnel boring machines, micro-tunneling, power shovels, steam shovels, trenchers, and various loaders.

Road or rail tunnel boring uses metal parts that are exposed to severe abrasion. Some tunnel boring equipment uses rings on the cutting face, while others use bits. The surface condition of metal parts and internal properties are critical in meeting the abrasion conditions of the work site geology.

For the many applications in the mining industry, All Metals & Forge Group supplies forgings for rings, gear blanks, step shafts, hybrid shovels, and crushers. AMFG has the know-how to meet the exacting requirements of the mining industry with forgings made from carbon steel and alloy steel. The company delivers parts to OEMS and aftermarket companies to suit their production schedules or repair demands.

Different forging techniques are used to achieve the final form for a metal part. Closed die forging or stamping is used for parts that are exact duplicates and high volume.  Casting can be used for volume production, but is often used for final shapes with various angles or curvatures. However, open die forging can be used for many cast parts to achieve the same end shape but with far superior mechanical properties, often at a similar cost.

Castings may be used for intricate shapes, providing the stresses on the finished casting will not involve too much load bearing. When the molten metal is poured into a mold, three crystalline shapes will form. The area in contact with the mold will form a chill zone of randomly oriented grains. Next there will be columnar grains, oriented perpendicular to the mold walls, and in the center of the casting will be found an equiaxed zone. Cast parts can be produced from a few ounces to several tons.

The origin of a forging is a casting, in this case as an ingot or billet from a steel mill. Ingots are cast in various shapes, square, rectangular, round or polygonal, as required. This is where the similarity ends in terms of chemistry and mechanical properties. Forgings are crafted quite differently to produce parts with more durable properties for their end use.

Castings of whatever shape will be subject to shrinkage upon solidification. There are methods of reducing the amount of shrinkage but the phenomenon will always occur. Shrinkage in low carbon steel, for example, will run to about 4%, that in aluminum to around 7%. Porosity is a common solidification defect in castings, and its presence will cause problems in load-bearing, corrosion, or abrasion applications. 

The grain structure of castings is non-uniform, hence heat treatment will not bring about the predetermined changes in properties noted in material that has been through a hot working process. But casting, in the hands of those who are steeped in it, might produce part shapes unattainable by any other method of production unless extensive finish machining is employed.

Open die forging picks up where casting leaves off, and will refine the different grain structures present in the cast ingot or billet through heating and then pressing of the material.  Open die forging brings about an inherent superior reliability, improved tolerance capabilities and better machinability. There is greater metallurgical soundness and improved mechanical properties imparted into the material. For the most part, forging stock has been preworked to refine the cast structure and remove defects or porosity from the casting process. This produces what is known as directional alignment, or grain flow, giving important directional properties in strength, ductility, toughness, and fatigue strength. Properly developed grain flow in forgings closely follows the outline of the final part. In contrast, bar stock and plate have unidirectional grain flow, and castings have no grain flow or directional strength.

The degree of structural reliability that is achieved in a forging is as good as, if not better than, that obtained from any other metalworking process. Chemical uniformity is achieved by dispersion of alloys that may have a tendency to segregate, as well as dispersion of nonmetallic inclusions (impurities) that might adversely affect performance of a part under load.

All metals can effectively be forged, and subsequently heat treated. The range of properties that may be obtained in forged products covers the entire range of ferrous and nonferrous metals. A forged part may undergo subsequent manufacturing processes, such as heat treating and specialized cooling, carburizing, or finish machining.

Forgings, with their uniform structures, will respond well to any chosen form of heat treatment. As such, a uniform range of required properties may be obtained from this combination of forging and subsequent heat treatment. Machinability itself will benefit from these carefully controlled processes. Forged parts are suitable for welding, machining of course, and for surface conditioning by plating, polishing or painting.

In many cases, the cost difference between casting and open die forgings can be modest with forgings sources from All Metals & Forge Group, an ISO 9001:2015 and AS9100D manufacturer of open die forgins and seamless rolled rings. Forgings generally cost more while presenting greater long-term durability that reduces replacement cost and total cost of manufacturing in machinery that requires replacement parts during maintenance cycles.

Time was when forging was just forging, beating heated metals into shape with the aid of a hammer. The principle is still the same, the methods infinitely more sophisticated. Today we have methods that have evolved to the point where forging can be controlled to produce complex shapes, large forgings, and seamless rolled rings. But the two major forging methods are called closed die and open die. The basic differences between these two methods control the sizes and shapes that they produce.

Typically, closed die forging is used when hundreds or thousands of identical parts are required with lower mechanical properties.  Closed die forging has size limitations because the parts are produced within a mold.  Open die forging is used when part volumes are just one to several hundred, and when significant structural integrity is required.

Forging techniques, although evolved, are still somewhat of an art, and benefit from an expert operator. This applies particularly to open die forging, where there is no complete enclosure of the metal, and repeated hammering and pressing can lead to the production of any desired shape. Open die forging involves two dies, an upper and a lower, that come into contact with the workpiece. As mentioned, the metal is free to flow where it may, and is effectively under the command of the operator. There are really no limits to the dimensions that can be produced by this process.

Grain flow in open die forging follows the shape of the part being forged. Working of the material leads to a final product that is consolidated, in that it has no porosity, has a uniform grain size, and good mechanical properties. 

The open die technique is best used for relatively small quantities, and is good for discs, forged and rolled rings, cylinders, and shafts, although more complex shapes can be achieved through forging expertis. It is also good for the production of oversize parts.

Closed die forging is not a free forging process, rather it is restricted to the shape of the dies. In fact, the process is often referred to as impression die forging, whereby the metal is forced into the die to give the final part its shape. There are repeated hammer blows and the dies, made normally from hot-work tool steels, will serve to produce the required components.

By the very nature of the closed die process, and the significant costs involved in manufacturing the part dies, the process is best used for long production runs.  The work that is put into die manufacture results in an overall ability to create tighter tolerances, and complex parts, hence less machining required. 

Both methods have inherent advantages. Open die forging can produce very large parts that would not be possible from the closed die process. Theoretically the closed die process can produce parts whose dimensional tolerances will surpass those from the open die process. Both processes produce a grain flow and a lack of internal porosity that result in required mechanical properties. 

As a manufacturer of open die forgings and seamless rolled rings, All Metals & Forge Group can produce a wide range of shapes and part sizes in various metals and heat-treated conditions.  Many industries rely on both open die forged parts and closed die forgings for construction of machinery, turbines, drive shafts, fluid ends, xmas tree flanges, rollers, and hundreds of other finish machined components.