Hardface Brazing

1      Introduction

Wear resistant coatings are used to protect metal parts operating in a hot gas environment from degradation caused by e.g. mechanical wear, oxidation and corrosion. Areas of application for such coatings are steam turbines and gas turbines. One specific example for the application of such coatings are surfaces exposed to the hot gas path and/or surfaces adjacent to hot gases, especially when there is physical contact to other surfaces.

Wear resistant coatings are typically applied by spraying, welding or brazing processes and often must be renewed after service. In cases where heavy wear occurs during service, reapplication of just the hardface coating is sometimes not sufficient rather restoration of the wall thickness is also required. As a consequence hardface brazing has emerged as a process where wall thickness build-up and wear protection can be achieved in one process step, thus providing cost savings compared to traditional spraying.

The hardface brazing process typically utilizes pre-sintered hardface sheets or hardface tapes, the green bodies of pre-sintered hardface sheets. The use of hardface tapes saves the process step of pre-sintering, leading to a more cost efficient product. However on the other hand, tapes require a different brazing cycle to pre-sintered sheets. Some recent optimizations of the hardface brazing process are summarized in the following sections. Section 2 describes the materials necessary for hardface brazing (sheets and tapes), whereas section 3 describes the hardface brazing process (sheets and tapes). The brazing cycle described in section 3.2 was specifically developed for brazing tapes.

2      Hardface Brazing Material

In order to achieve wear protection by brazing, the braze alloy is mixed with wear resistant particles. Additionally, filler and binder material may also be added.

2.1    Wear Resistant Particles

Wear resistant particles are preferably ceramics, e.g. carbides, carbonitrides, nitrides and oxides. Some examples of wear resistant particles include but are not limited to:

Boron nitride, chromium carbide, chromium oxide, diamonds, silicon carbide, silicon nitride, tantalum carbide, titanium carbide and tungsten carbide. Depending on the application, a mixture of one or more types of wear resistant particles may be used.

Other important parameters that define the wear properties of the wear resistant layer are particle shape, particle size and particle size distribution. Generally smaller particle sizes are preferred if the brittleness of the hardface coating is a concern. Particle sizes below 100 mm or even below 50 mm are frequently employed. Various particle size distributions are possible, such as but not limited to, Gaussian or bimodal distributions. Bimodal distributions are often used when more than one type of wear resistant particle is used with the aim to tailoring the properties of the layer.

The above-men...