Studies on Boronization of Steel Surfaces by B₄C-NaOCN

Abstract

A new surface hardening method using B₄C-NaOCN has been developed. According to the new method, the steel specimen is immersed into a NaOCN bath (560°∼570°C) to from thin film of NaOCN on the surface of the specimen, and the specimen is then embedded in B₄C powder and heated in a closed air furnace at 900°∼1100°C. This method can be successfully applied to the surface hardening of pure iron, plain carbon steels and alloy steels.
The thickness of the boronized surface layer is 0.1∼0.3 mm and depends on the treating time and temperature, especially the temperature: The Vickers hardness is as high as 1400∼1800 for the specimen heated for 5 hr at 1000°C: The wear resistance is quite good and the corrosion resistance to H₃PO₄ and H₂SO₄ is as good as cemented carbide alloys. However, the corrosion resistance to HCl, HNO₃ and H₂O is not so much improved as compared with the non-boronized steel. The boronized layer consists of FeB and Fe₂B, and a comb-shaped figures at the interface between the boronized layer and mother alloy are observed.