Cast steel
Cast steel is the starting material for castings made of steel (in contrast to castings made of other cast materials and ingot and / or continuous castings). Unalloyed cast steel includes iron-carbon alloys with a maximum of 0.60% silicon and up to 1% manganese, the carbon content of which, up to 0.5%, significantly determines the strength properties. Low to high-alloy cast steel also contains varying proportions of alloying elements such as chromium, nickel, molybdenum, vanadium, tungsten and others. In the case of steel casting, the advantageous properties of steel and the design advantages of the casting technology are combined in the end product (steel casting). Most forged steels are also cast into cast parts, but are then provided with a GE or G (formerly GS) in front of the steel brand name (example: G42CrMo4).
Cast steel is significantly more demanding to manufacture than other cast iron materials such as cast iron for two reasons :
- Cast steel has a higher casting temperature (approx. 1600 ° C) than cast iron (approx. 1150 ° C). These increased temperatures place greater demands on the melting technology, the refractory materials of the furnace cladding, the crucible and casting tools, and finally on the molding materials .
- With cast steel, the shrinkage of two percent is around twice as great as with gray cast iron.
- Since the cast steel pieces are brittle, coarse-grained and dendritically solidified in the as-cast state , these parts must be subjected to a heat treatment ( normalizing , quenching and tempering, soft annealing, stress-relieving annealing).
Due to the large difference in the specific volumes of the material just below the solidification temperature and at room temperature, cast steel tends to form more cavities than cast iron, and a higher degree of shrinkage must also be taken into account. Without special countermeasures ( feeders ), cast steel parts would become unusable due to blowholes or would be unprofitable to manufacture with extensive production welding. The feeders on cast steel parts are removed with oxy- fuel cutting by burning and removing the material in the so-called kerf using the heat of oxidation from the supplied oxygen. For smaller riser diameters and special steel alloys, tee feeders are preferred. Because of the mechanical-thermal rinsing effect of the steel stream, ceramic inserts (chamotte) are used ( gate ) to increase the surface strength in the pouring system of the larger molds . By means of firing joints (gouging), the material is further trough-shaped to expose and remove casting defects and to model the surfaces and prepare it for any repair welds.
The highly hypoeutectic composition of the steel alloys leads to a very viscous melt and therefore to poor shape filling capacity , which means that fine structures can only be produced by subsequent machining.
On the other hand, products made from cast steel have better mechanical properties ; cast steel is ductile and weldable. All common types of steel can be used, including stainless steels .
Large steel castings can weigh several hundred tons, e.g. B. Housing for steam turbines .
For future high-temperature reactors (HTR), prestressed vessels made of cast steel or nodular cast iron are being considered as reactor pressure vessels .
Jacob Mayer, who was born in Dunningen , made the first successful attempts to cast complex steel parts in one piece as technical director of the Bochumer Verein in 1841. He had his process patented on December 16, 1851.
Experiments at the Helmholtz Center Dresden-Rossendorf show that magnetic brakes can specifically influence melting in steel casting. The quality of cast materials can be increased if externally applied magnetic fields stir, slow down or calm the molten metal, which is still liquid.
Machining allowances
The machining allowance is determined depending on the largest dimension of the casting and the casting (molding) process used. The standards for this material provide specifications, but the applied values are based on the experience of every steel foundry. For surfaces that form the upper limits in the mold, the fixed values of the machining allowance are increased by 2 to 10 mm. When used for weld-in parts, the allowances should be selected 20 to 50 percent larger in order to be able to compensate for the welding deviations on the finished part. Bores and grooves, which are to be regarded as small in relation to the cast part, are often fully cast and made by machining. In order to deliver the cast in accordance with quality, all areas of the drawing with machining marks are usually delivered by the foundry with a cutting allowance of 3 to 8 mm per area. Dimensions that are not tolerated should be agreed in the tolerance ranges DCTG 11 to 14 using DIN EN ISO 8062-3.
|
Nominal size range (greatest length, width, height or largest diameter of the casting) in mm |
Reference value for addition per area in mm |
| until 50 | 4-5 |
| 51-180 | 6th |
| 181-315 | 7th |
| 316-500 | 8th |
| 501-800 | 10 |
| 801-1250 | 12 |
| 1251-1600 | 14th |
| 1601-2500 | 16 |
| 2501-3150 | 18th |
| 3151-4000 | 20th |
| 4001-6300 | 25th |
| 6301-10,000 | 30th |
standardization
- German standard cast steel for pressure vessels DIN EN 10213; January 2008
- German standard corrosion-resistant cast steel DIN EN 10283; December 1998
- German standard cast steel for general applications DIN EN 10293; June 2005
- German standard for heat-resistant cast steel DIN EN 10295; January 2003
- German standard cast steel for construction DIN EN 10340; January 2008
Individual evidence
- ↑ Helmholtz Research Center Dresden-Rossendorf ( Memento from April 29, 2013 in the web archive archive.today )