Stainless Steel Heat Treatment (SolNit Process)
Introduction
The development of the SolNit solution nitriding process in the late 1990s resulted in an
extensive partnership between Ipsen and Hîrterei Gerster. This collaboration led to the
development of two innovative industrial processes for hardening stainless steels, namely
SolNit-A and SolNit-M.
SolNit-A for austenitic steels increases cavitation resistance by creating high compressive stress
in the surface zone of a component. SolNit-M for martensitic steels enhances the surface
hardness and related properties, while maintaining a relatively ductile core and improving
corrosion resistance.
The SolNit processes extend the use of low-grade stainless steels from their typical markets to
fields demanding special surface, magnetic, electrical, and low-temperature properties.
Fundamentals of Process Technology
Typically, carburizing and nitriding of high-alloyed stainless steels is not possible at the
temperature range of 500-1000°C, due to a reduction in corrosion resistance, which is caused by
the formation of chromium carbides and chromium nitrides due to the low solubility of these
steels.
Reducing the carburising or austenitising temperature to a point at which precipitations are not
produced during the treatment time is one approach used to prevent the formation of chromium
carbides and chromium nitrides. However, low-temperature processes only generate thin surface
layers. Conversely, the advent of the SolNit process enabled the formation of thicker diffusion
layers.
The process is based on the fact that the higher proportion of molybdenum, manganese, and
chromium increases the nitrogen solubility of the steel at temperatures above 1050°C. Figure 1
illustrates the variations between the typical limits of chromium content in stainless steels and
the regime of homogeneous austenite in nitrogen-alloyed steel, demonstrating that the difference
is greater and extends to a higher interstitial content when compared to carbon-alloyed steel.
Figure 1. Isothermal sections of equilibrium phase diagrams at (1100°C)
The austenitic process forms a corrosion-resistant case with high strength, ductility and nitrogen
content. These characteristics offer resistance against surface fatigue in aggressive media. An
additional mechanical solidification of the surface produces compressive stresses and increases
fatigue strength.
The cavitation resistance is also strongly improved by this process, which forms an aggressive
media usable for application in fluid-flow machines, including turbines, pumps, and the
associated armatures.
The martensitic process forms a combination of hard case tough core. The nitrogen increases the
corrosion resistance in media comprising chloride and acid. The case has high compressive stress
and a hardness of 58-60HRC.
The SolNit Process Technology
The SolNit process technology is a relatively simple process because all-metal surfaces contain
two-atomic nitrogen molecules, which dissociate into atomic nitrogen at temperatures beyond
1050°C. The dissociated atomic nitrogen increases the nitrogen content of the steel by
penetrating into the surface, despite the material’s passive surface under an oxygen-free furnace
atmosphere.
The three factors that influence the yielded surface nitrogen content include the temperature, the
partial pressure of the nitrogen and the alloyed contents of the stainless steel. The equilibrium
diagram shows the solubility limit of the nitrogen austenite needed for the highest possible
surface nitrogen content. Thermo-Calc software is used to determine the equilibrium diagram for
each steel type.
The coordination of alloying element content, temperature and pressure is essential to dissolve
adequate nitrogen and to prevent precipitation of nitrides (Figure 2). The typical process
conditions for the SolNit process include a temperature range of 1050-1150°C, nitrogen partial
pressure range of 0.1-3bar abs., and diffusion times from 15min to 4h. The nitrogen depth
achieved is in the range of 0.2-2.5mm. The surface hardness of martensitic steels is between 54
and 61HRC, and for austenitic and duplex steels between 200 and 350HV.
Figure 2. Precipitation of nitrides in dependence on nitrogen content, temperature and nitrogen
partial pressure for martensitic steel
The quenching step is critical in the SolNit process. The solubility of the austenite declines when
the temperature decreases. Hence, the quenching process must be very fast to avoid the
precipitation of chromium nitrides. This can be achieved with high-pressure gas quenching with
rapid gas flow, or quenching in oil.
A nitrogen-containing martensite with large retained austenite content is produced by the rapid
quenching of martensitic stainless steels. It is possible to reduce the retained austenite content by
deep cooling and tempering at temperatures of up to 450°C, thereby achieving high surface
hardness values. However, the grain growth at these high-nitriding temperatures can cause
certain problems. Refining the grain size by a double-hardening treatment can yield high-quality
toughness.
Figure 3. Typical hardness profiles after SolNit-M treatment
The hardness improvement is significantly lower in austenitic steels because the grain coarsening
cannot be reshaped by thermal processes. The grain structure in the core is relatively stable in
two-phase austenitic-ferritic steels such as duplex steels.
The typical hardness profiles for the corrosion-resistant, martensitic steels after a SolNit
treatment are depicted in Figure 3. The core hardness values rely on the amounts of ferrite,
chromium, and carbon of these steels, and are between 220 and 510HV.
Figure 4. Sample with a ø 2.5 mm hole. The measuring points are marked
A sample with an open hole (Figure 4) was analyzed to determine the degree of nitriding of
scoring, grooves, and blind holes. A hardness profile of the entire wall thickness was measured at
three defined measurement points distributed across the sample length (Figure 5).
The results conclude that nitriding conditions exist on the external and internal diameter are the
same. Moreover, homogeneous nitriding is possible on open and blind holes. However, the limits
regarding L/D ratios must still be studied. The atmosphere during solution nitriding is less
volatile and non-toxic when compared to carburizing processes. This is because the process does
not require continuous gas flow or internal oxidation.
Figure 5. Hardness profile of three measuring points
Applications of the SolNit Process
The SolNit-M process is used in the following fields:
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Chemical industry
Textile processing
Machine Building industry
Architecture
Medical industry
Household and kitchen appliances
Food processing industry (e.g., milk and dairy products)
Conclusion
Like case hardening, SolNit is a heat treatment process, but utilizes nitrogen in place of carbon as
an alloying element. The industrial SolNit process employs vacuum furnaces with high-pressure
gas quenching for nitriding stainless steels. The following are the advantages of the SolNit
process over standard case hardening:
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Increases surface hardness without compromising the corrosion resistance properties
Avoids the formation of metal carbides in the steel
Increases high-temperature strength
Improves resistance against cavitation, erosion and wear
Use of nitrogen as a process gas makes SolNit as a safe and environmentally friendly
process
Source : By AZoM.com Staff Writers