Importance and potential of knife engineering
In many applications of the technology industry, various materials are cut to size. When cutting certain materials, problems are often encountered that can adversely affect the service life of the industrial knives used. In this search for solutions, three important topics largely determine the function and service life of the knives: the choice of material of the knives, the corresponding heat treatment and the surface treatment of the knives. Obviously, many properties are interrelated, all aspects must be considered in order to arrive at an optimal knife.
Surface hardening versus massive hardening
For certain applications, massive hardening of blades is required, e.g., when there is a lot of wear and tear and frequent re-sharpening. In other cases, surface hardened layers are a possibility. Thermal surface hardening is often applied to steels that contain sufficient carbon, namely from 0.25 % C. The hardness achieved by the hardened layer is again determined by the carbon content, while the hardness of the core remains unchanged. The advantage of this technique is that the heat input is limited, so that long pieces undergo little deformation, as opposed to massive hardening. The most common technique is induction hardening, using special inductors. After heating, the pieces are quenched in an aqueous polymer
quenchant. Such a method can also be carried out in a press machine, whereby the flatness is guaranteed (press hardening). Flame hardening is also still used, but is particularly suitable for very large parts, where the accuracy of the hardening depth is less important.
Induction hardening is better in terms of process control, as the depth of hardening is inversely proportional to the frequency of the induction field. A newer technology with growing application is laser hardening. As with other laser processes, this type of hardening is very precise, however, the hardening depth is usually less than 1 mm, depending on the geometry. Research has shown that knife points lend themselves well to laser hardening (see Figure 1). A disadvantage is that good temperature monitoring is needed to prevent melting. The laser-hardened layers are also very hard, as the tempering can be omitted. After all, the process is 'self-quenching' as well as self-tempering, as the residual heat in the piece creates a
certain stress release effect. The limited track width of the laser beam must also be considered: sometimes several tracks have to be laid side by side. The study showed that the surface hardness over these tracks is fluctuating due to tempering effects between the tracks.
Figure 1: Example of a laser-hardened knife, left cross-section, right blade (Source: Sirris)
An additional advantage of laser hardening is its very low distortion, as the heat supply is very localised.
Diffusion Processes Versus Surface Hardening
The above methods are thermal processes: only heat is supplied, not material. In the so-called “diffusion processes” such as carburising, boronising, inchromising and vanadising, foreign elements - respectively carbon, boron, chromium and vanadium - are diffused into the material at a high temperature. This high process temperature usually causes deformation after cooling, so this must be considered. A dimensional change (swelling) may also occur because of the addition of material to the surface. The hardness obtained depends on the %C after hardening and is usually around 60 HRC at approx. 0.7 - 0.8 % surface C. As with the other processes, the surface hardness is high, but it decreases as one goes deeper into the material
Figure 2: Hardness profiles of different surface treatments on steel as a function of depth
You can see that the curve of induction hardening at the transition zone to the core is steeper than in carburising. This transition zone can be reduced by first applying a pre-hardening, which increases the hardness of the substrate and reduces the difference between the surface and the core. With carburising, the transition is more gradual, as the hardening transforms both the surface and the core.
There are also diffusion processes at lower temperatures such as nitriding and variant methods. In contrast to carburising (heating in high carbon environment and hardening), gas nitriding (adding nitrogen from gas atmosphere) or nitrocarburising (adding nitrogen and carbon) is applied at temperatures below the austenite phase (ferrite phase), which limits the deformation after cooling. Quenching is also not necessary like with the other processes, because nitriding is usually applied to already hardened substrates as a final treatment. However, the thin nitriding layer (approx. 0.1 - 0.3 mm) must be well supported by a hard core to avoid the ‘eggshell’ effect.
However, nitriding is rarely used for knives and the reason is probably the fact that nitriding layers do not
perform well under a localized contact load. This is because the nitriding layer is brittle, as it consists of
iron nitrides (‘alpha and epsilon nitrides’). For cutting operations with punching (cutting), this technique is
used, namely as an intermediate layer for a PVD layer ('duplex coatings'). Plasma nitriding, instead of gas
nitriding, is usually used for this purpose. Research shows that this greatly improves the adhesion of the
PVD layers. With this technique, so-called 'adhesive wear' (or cold welding) is avoided due to the improved
Figure 3: PVD-coated blades for grinding coffee beans
Thermally applied layers versus diffusion layers
In diffusion layers, a foreign element (e.g., C, N or both) is usually introduced into the surface of the
material in the form of a gas. Such a diffusion process is slow (hours to days) and requires high
temperatures (except for nitriding), with the layer thickness usually limited to 1-2 mm. The application of
thicker layers with high hardness is better done by thermal spraying or by thermal cladding. The
advantage is that the substrate is hardly affected by the heat input and the choice of materials is very
extensive. A disadvantage is that the size of the parts changes due to the external application of material to
the surface. For some applications, such as heavy shredding knives, this method is appropriate, as the
cutting edge is usually not sharpened to a knife edge.
In this section, however, we would like to explain the importance of laser cladding. As with laser
hardening, heat is transferred very locally by a laser to the substrate, whereby the material is added in
powder or rod form (Figure 4).
Figure 4: Principle of laser cladding
Several welding traces are required to cover large areas. A widely used alloy for applications with high wear
resistance is NiCrBSi. This alloy contains chromium carbides that ensure wear resistance, while the presence
of Ni provides good corrosion resistance. Sometimes it is necessary to preheat the material in order to
achieve good adhesion without cracking. Cobalt-based stellites and nickel coatings with tungsten carbides
as hard phases are also used.
Thermal spraying with HVOF (High Velocity Oxygen Fuel), unlike welding, is a cold technique (with no
influence of heat on the substrate) and has greater flexibility for coating complex shapes and parts.
Hardening of stainless steel knives
Hardenable blades in stainless steel usually consist of 'martensitic' stainless steel, such as X40Cr13 steel.
Such steel can be hardened in the core as well as at the surface. However, there are also knives that must be
made of austenitic stainless steel, such as AISI 304/316, because of the required corrosion resistance.
However, these steels are not hardenable, as the carbon content is too low and the structure remains
austenitic after quenching. The surface of these steels can still be hardened by special diffusion processes
(both carburising and nitriding). Both processes are carried out in a plasma reactor under very strict
control of temperature (approx. 400 °C) and pressure, whereby corrosion resistance is maintained. So-called
'kolsterising’ process is based on the classic process of low temperature carburising treatment, but under
very specific conditions. The layers obtained are usually thin, 20-30 μm, but very hard. Surgical blades with
very high cutting precision are hardened in such a way. The process is relatively expensive compared to
other processes but seems to be the only possible hardening method for stainless steel.
Figure 5: Cross-section of a kolsterising layer on stainless steel and hardness profile (Bodycote)
An analogous method based on gas nitriding in a vacuum oven is the SolNit process and the Nitruvid
process based on plasma nitriding (at 1,050 °C). The 'Expanite-LowT' and 'Expanite HighT' are combined
processes of nitrocarburising at a high temperature and a low temperature respectively, whereby a high
hardness is achieved while maintaining the corrosion resistance.
Hard coatings such as hard chromium and chemical nickel (Kanigen process) can be used in some cases if
the cutting edge does not have to remain too sharp. After all, as soon as the knife tip wears off, the layer
will usually lose adhesion and peel off. The advantage of these chemically applied layers is that the
substrate is not thermally influenced and that the layer thickness is very uniform. In the case of nickel
layers, additives, such as Teflon, are also possible in the layer, which can ensure a certain lubrication.
However, the layers are usually too thin (20-100 μm) to guarantee a long service life.
Which method to choose?
Of course, each method has advantages and disadvantages (see table below). There is also a difference in
the cost of treatment. Diffusion processes are generally more expensive than thermal surface treatment due
to the long process times.
Sirris can carry out several hardening processes and give advice on the most optimal surface treatment for industrial knives. Sources
- Knife Engineering Steel, Heat Treatment and Geometry Dr Larrin Thomas
- Heat treatment course VWT
- Advanced Surface Technology P. Moeller, L.P. Nielsen
- Heat Treating Stainless Steel (www.ipsenusa.com)