Bright plasma nitriding of ferritic steel with several alloying elements
- PDF / 401,182 Bytes
- 6 Pages / 612 x 792 pts (letter) Page_size
- 74 Downloads / 250 Views
Y5.43.1
Bright plasma nitriding of ferritic steel with several alloying elements J.-D. Kamminga, C. Kwakernaak1, G.C.A.M. Janssen1, Netherlands Institute for Metals Research, Delft, Netherlands, e-mail: [email protected]; 1 Department of Materials Science and Technology, Delft University of Technology, Netherlands ABSTRACT Hot work tool steel was plasma nitrided in a PVD apparatus. Phases occurring and nitrogen depth profiles were assessed for the nitrided specimens. In all cases bright nitriding was achieved. Increasing the nitriding time from 15 to 120 min yielded a nitriding depth increasing from about 15 to 35 µm, whereas the nitrogen flux decreased from 8.0 to 5.5 x 10-7 kg s-1m-2. Results from an existing model, predicting nitrogen depth profiles due to plasma nitriding, did not accurately describe the experimental data. Therefore the existing model cannot be used straightforwardly to predict the formation of iron nitrides. The discrepancies of model results and experimental results are discussed in view of the assumptions underlying the model for plasma nitriding. INTRODUCTION Deposition of duplex coatings, that combine high hardness and good wear properties of PVD coatings with enhanced fatigue resistance and load carrying properties of the hardened nitrided steel substrate, is not without complications. During nitriding of the steel substrate the iron nitrides Fe4N and/or Fe3N, often referred to as white layer, easily form. These iron nitrides are unfavourable for the adhesion of the coating to the nitrided substrate [1,2,3]. Therefore, the iron nitrides should be removed before the PVD treatment, or the formation of iron nitrides during nitriding must be avoided. In pure iron, iron nitrides form if the nitrogen concentration exceeds a critical value. The same critical value holds for nitrogen in solution in steel. However, not all nitrogen is in solution, because some of the nitrogen forms nitrides with the alloying elements. Therefore, in steel the total amount of nitrogen necessary to form iron nitrides can be much higher than in pure iron. The amount of nitrogen in solution, governing the formation of iron nitrides, depends on the total nitrogen content, the amount of alloying elements and their chemical affinity to nitrogen. The various alloying elements can be subdivided into groups of strong (Ti, V), intermediate (Cr, Mo), and weak (Al, Si) nitride formers [4,5]. A model predicting the formation of iron nitrides and the nitrogen depth profiles due to (plasma) nitriding on the basis of alloying element - nitrogen interaction, for steels with several alloying elements, has been proposed in Refs [6,7] and elaborated in Ref. [8]. According to this model, the formation of alloy-nitrides is governed by the product of the amount of alloying element in solution in the steel and the amount of nitrogen in solution in the steel. If this product exceeds a critical value K, the equilibrium solubility product, - different for each alloying element - alloy-nitride forms (leading to a reduction of both th
Data Loading...
