Main precautions in the design of steel parts

Ferrous materials are widely recycled. Nearly half of steel production in Europe comes from recycled scrap and recycled steel and return in noble applications such the mechanics or the building.
The sort of conventional steels is usually done by magnetization of pieces cruhsed after grinding of the product.
The presence of residual elements such as copper, tin, alloy elements such as chromium, nickel, manganese, molybdenum or aluminium parts brings the complexity of recycling, which results in either a degradation steel (the presence of copper or tin) and a loss of raw materials (nickel, chromium, molybdenum, aluminium …)

So one should avoid close associations with other materials, with emphasis on associations of steels of the same nature, to avoid degradation of recycled steel or raw material losses. Otherwise, it will facilitate the separation of materials by:

– Preliminary identification and removal of the product;

or

– The separation and sorting of materials at a grinding phase of the product.

Rules of association for alloying elements with steel

Impacts of alloying elements (B. Castro & al. – 2004):

Criticality:

• Red: High criticality, strongly harmful impacts.
• Orange: Medium criticality, harmful impacts.
• Green: Low criticality, no harmful impacts.

Avoid closely associated steel with copper or tin.

The main problem of recycling of steel is due to the presence of copper in the scrap. Copper reduces the mechanical properties of steels and can not be eliminated by metallurgical processes. It therefore tends to accumulate with each cycle of recycling scrap metal and could eventually condemn the use of noble scrap.

Usinor Bernard Gros 2000
Copper accumulation in scrap

The main design action will be to avoid tangling of copper alloys with ferrous metals which could not be segregated during the shredding of the product (see sample items below).

Engine before and after passage through a shredder. The mild steel frame encloses the copper	The fins brass pump water are folded and have not been separated at the time of grinding (ductility of brass).

Tin has the same effect of reduction in mechanical properties of steel and can not be eliminated by metallurgical process. It is therefore necessary to avoid associations with steel (such as in the case of tinplate)
In addition, copper and tin, are materials that become scarce will be definitively lost.

Facilitate removal and identification of special steel parts

Other rare elements such as chromium or nickel (stainless steel), Molybdenum (heat resistant steels), Manganese (special steels) are associated with the steel to bring specific properties. The magnetic properties used to sort the steel may be modified by the presence of alloying elements (eg stainless steels) and their recovery by magnetic separation will be inefficient and require manual sorting of the product after grinding. Given the value of these metals, it should be able to disassemble and sort them before the final grinding of the product (pending reliable spectroscopic sorting techniques after grinding). Should be avoided to closely associate these with other ferrous metals. A visual marker on the back of the parts indicating the type of alloy used would be appropriate.
 

Taking into account the limitations of current recycling processes

The compressor of refrigerator: a particularly difficult part to recycle

Some design habits of metal parts can lead to problems of breakage or maintenance of recycling facilities, and induce additional sorting steps.

Case of very thick plates or heavy parts

The pieces of sheet thicknesses greater than a few tenths of a millimetre, or the presence of massive pieces of steel in products such as motors (washing machine, refrigerator compressors or large bolts, ball joints) may cause a deterioration of conventional grinding facilities and require the use of automotive shredder (with hammers), with a capacity of several hundred CVs. It should therefore be avoided in the manufacture of SHA or LHA the presence of large pieces of steel that can damage other shredders.

Heavy metals and cleanness of scrap

The presence of plastics in the scrap metal, plastic coatings for parts such as film, paint or adhesives, oils or other organics generate extra pollution that requires producers to capture oversize electric furnaces and therefore generates economic and ecological additional costs. From a design point of view, to facilitate the draining of fluids, it would be appropriate to carry tanks or sumps with a low point. A drain plug may be added, or a default marking the low point of the housing allowing the operator to break into the housing to the right place for an oil change. It will also avoid associating closely with metal or plastic materials of organic coatings, or coatings containing heavy metals (Pb, Cd, Cr VI, etc.).

Synthesis

The design principles of steel parts can be synthesized by the following recommendations (not exhaustive):

• Avoid associating intimately steel parts with copper alloys or coatings or Tin parts. Otherwise, think of ways to design non-encapsulating copper or tin and use of ferrous materials are more fragile than copper.
• Do not apply within the same product different grades of specialty steels. Think about the dismantling and marking type of alloy for large special steel parts.
• Avoid heavy parts that would damage the mills.
• Avoid close associations of coatings or materials based on plastics / metal parts / elastomer.
• Avoid surface treatments containing heavy metals.

References

– B. Castro, H. Remmerswaal, U. Boin and M.A. Reuter (2004): A thermodynamic approach to the compatibility of materials combinations for recycling. Resources, Conservation and Recycling, Vol. 43(1), pp. 1-19

– M.A. Reuter and A. van Schaik (2008): Thermodynamic Metrics for Measuring the “Sustainability” of Design for Recycling. Journal of Metals, Vol. 60(8), pp. 39-46.