Corrosion resistant stainless steel is widely used in industry and trade due to its outstanding properties. Despite its numerous advantages, particularly its resistance to corrosion in numerous media, stainless steel also has considerable weaknesses that sometimes severely limit its application.
One of these weaknesses is its susceptibility to cold welding or galling, which can occur particularly with screw connections. Surface hardening offers a solution to this problem.
Improving Galling Resistance on Stainless Steel
Galling can be favoured by various aspects, such as insufficient surface hardness, excessive roughness or lack of lubricants. Nevertheless, fasteners are essential in many applications to allow for particularly frequent assembly and disassembly, or high tightening torques. It is therefore important to carefully tailor the properties of such elements.
When fasteners are jammed due to galling, their removal can turn out to be an enormous challenge, especially if it needs to be done without damaging the affected bolt or destroying the associated nut. Surface hardening, a process in which only the surface layer (few to tens of microns) of a workpiece is hardened, has been proven to significantly improve the seizure behaviour of stainless steel screws. The diffusion-based process increases the hardness of the surface through solid solution hardening and subsequently reduces its susceptibility to plastic deformation. Depending on the material, the corrosion resistance is maintained or even improved, and the stiffness of the base material is not impaired
The process
Surface hardening is not an applied coating, but a diffusion-based thermo-chemical surface hardening process. Although classic processes for the surface hardening of corrosion resistant stainless steels have been available to the industry for some time, their application has two main disadvantages. Firstly, all classic processes such as salt bath processes are known to reduce corrosion resistance. On the other hand, the hardness values of stainless steels usually hardened with nitrogen or carbon drop very quickly, i.e the hardening depth is only a few micrometers and the underlying base material is very soft, which can potentially lead to the ‘eggshell’ effect.
In contrast, Expanite has developed its proprietary technology SuperExpanite®, having solved those shortcomings as a priority. With this technological process not only the outermost layer but also the underlying material is hardened to a greater depth, which also contributes to an improvement in corrosion resistance in many cases.
A two stage process is used in Expanite’s treatment for stainless steels, in which nitrogen is introduced deep into the surface area in the first step called ExpaniteHigh-T (high temperature process), whereby the material is hardened to approximately 300HV for austenitic materials and 850HV for alloys with martensitic structure to a depth of up to 1mm. In the second process step, ExpaniteLow-T, the workpiece is heated to temperatures below 500°C and the surface layer is hardened to 1,100HV – 1,300HV at a hardening depth of 5μm – 30μm (depending on the solution). The hardening is achieved by incorporating large amounts of interstitial carbon and nitrogen.
By combining the two process steps, the so-called ‘eggshell’ effect, which occurs in most surface hardening processes, is avoided. This offers a decisive advantage for numerous applications that require improved load-bearing capacity.
Expanite has achieved remarkable outcomes through the application of SuperExpanite® hardening on AISI 316/1.4404 stainless steel intended for metallic food contact material. The testing, conducted in line with the Council of Europe Resolution CM/Res(2013)9, specifically targeted scenarios involving repeated food contact. Consequently, Expanite’s surface hardening process has been deemed safe for use in food contact applications.
The test results underscore the exceptional performance attained through Expanite’s surface hardening technique. The treated AISI 316 stainless steel showcased outstanding resistance and adherence to the rigorous conditions stipulated in the CM/Res(2013)9 test.
The test
The tests was conducted in accordance with the Council of Europe Resolution CM/Res(2013)9, focusing on repeated food contact scenarios. The CM/Res(2013)9 is a stringent standard that evaluates the compliance of materials with all types of food, under various contact conditions. These conditions include heating up to 70 °C for up to 2 hours or up to 100 °C for up to 15 minutes, without prolonged storage at room temperature or in a refrigerated state.
Test parameters
Material: AISI 316 Stainless Steel
Expanite treatment: SuperExpanite20 NC
Testing Medium: 0.5% Citric Acid
Temperature: 70 degrees Celsius
Duration: 2 hours
The test results
The tests results show the exceptional performance achieved through Expanite’s surface hardening on stainless steel – the treated AISI 316 stainless steel exhibited remarkable resistance and compliance with the stringent conditions outlined in the CM/Res(2013)9 test. In the larger perspective, it means that it’s considered safe to apply Expanite surface hardening to on stainless steel – and thereby combining this safety with superior wear resistance.
The surface hardening and heat treatment applied by Expanite not only meet but exceed the expectations for materials used in repeated food contact applications. The enhanced properties of the treated stainless steel make it an ideal choice for various industries, including those requiring compliance with strict regulatory standards.
Although this regulation came into effect in 2007, it gained more attention in 2013 when Chromium Trioxide (Chromium(VI) oxide) was added to the list of substances requiring authorization because of its carcinogenic and mutagenic properties.
Chromium(VI) oxide stands out from other substances on this list because of its wide range of applications. It is primarily used for surface treatments, whether for enhanced corrosion protection or the production of visually attractive surfaces. As a result, many industries, such as the electroplating industry and mechanical engineering, are feeling the impact of this regulation.
Coating
In general, coating methods that can prevent corrosion tend to add material and are often inconsistent in terms of the thickness of the applied layer. This can result in material accumulations on flat structures or material thinning at corners and edges where protection is reduced. Additionally, coatings may crack and peel off during significant expansion or contraction of the base material due to differences in the thermal expansion coefficient between the base and the coating. Complex contours with undercuts and small openings pose additional challenges, and certain coating methods are not compatible with these surfaces. Expanite takes a different approach to this process that avoids these issues.
The sustainable Expanite® processes
A solution to the above-mentioned problems involving stainless steels is surface hardening. This is not a coating process but a diffusion-based surface hardening process. Traditional methods for surface hardening corrosion-resistant stainless steels have been available to the industry for some time. However, they mainly have two disadvantages: they reduce corrosion resistance, and the hardness value of steels hardened with nitrogen or carbon often decreases rapidly at the material core. This means that the hardening depth is only a few microns, and the underlying structure is very soft, which can potentially lead to an “eggshell effect.”
Expanite has developed a method that hardens not only the outermost layer, but also hardens the underlying material to a greater depth. In many cases, this also results in improved corrosion resistance.
The Expanite treatment for stainless steels involves a two-step process. In the first step (high-temperature process), nitrogen is introduced deep into the surface zone, allowing the material to achieve a hardening up to 1mm in depth ranging from approximately 300 HV (austenitic materials) to 850 HV (martensitic materials). In the second step, the so-called low-temperature process, the workpiece is heated to a maximum of 470°C, and the surface layer is hardened to a depth of 5-30μm, achieving 1100-1300HV (see photo 1), through the embedding of carbon and nitrogen. Combining these two processes prevents the aforementioned “eggshell effect” and offers significant advantages for different kinds of applications.
Eco-Friendly
Despite its widespread use, hard chrome plating has several disadvantages. Harmful chemicals such as chromic acid, sulfuric acid, lead, lead chromate, and lead oxide are all used in the process. Additionally, it is largely inefficient with at least 70% of the electrical energy being wasted.
SuperExpanite®, the trade name for the layer produced in the Expanite’s two-stage process, stands out for its resource-saving approach. It neither consumes natural re-sources nor produces polluted wastewater. With a low energy consumption of only 1.9 kWh/kg of hardened stainless steel and without the use of harmful chemicals, it offers an environmentally friendly and sustainable alternative to hard chrome plating, which also extends the lifespan of fittings.
The nitrocarburizing process developed by Expanite enhances both the hardness and corrosion resistance of stainless steel materials – and not just for individual alloys. Therefore, the process is also recommended for medical device applications. It extends the lifespan of the respective application, thereby increasing patient safety and reduces cost.
Because of its unique properties and versatility, Stainless steel is an essential material for medical devices and implants that frequently encounter liquids and tissue. The corrosion resistance of stainless steel guarantees an extended lifespan and prevents the development of rust or other forms of damage that could compromise the functionality and safety of medical products.
For decades, corrosion-resistant stainless steel has been extensively used in the field of medical technology and is offered in a diverse array of alloy variations in the market. Numerous companies opt for these materials due to their corrosion resistance, with particular emphasis on the austenitic and duplex variants. Nevertheless, , these options are challenged by their susceptibility to wear attributed to their relative low hardness, often resulting in issues such as galling.
Ramén Valves, a leading manufacturer of industrial valves for the process industry, has announced its commitment to a more sustainable future by eliminating the use of hard chroming in its manufacturing processes. Instead, the company has adopted the more environmentally friendly SuperExpanite® treatment, a gas-based process that yields no toxic waste and has the lowest carbon footprint in the industry.
Upcoming regulations spark the need for more environmentally friendly surface hardening processes
Hard chroming is a process that increases the hardness of parts in contact with abrasive media, making it a popular solution in the valve industry. However, with the upcoming European Commission’s REACH Committee’s agreement to eliminate the use of chromium trioxide in most manufacturing processes by September 2024, many companies have been seeking alternative solutions. While some hard chroming companies have been promoting the use of alternative chemical solutions, Ramén Valves has decided to go a step further and eliminate all hard chroming solutions.
“We are committed to protecting people and the environment from harmful chemicals, whether they are currently allowed or not. That’s why we have been researching alternative solutions for many years now and have found a successful solution in the SuperExpanite® treatment.” says Per Wennersten, CEO, Ramén Valves.
A gas-based process with no toxic waste
SuperExpanite® is a patented surface hardening technology, invented by the Danish company Expanite, that increases the hardness, wear-resistance, and improves corrosion resistance of stainless steel and titanium components. The treatment is performed without the use of harmful chemicals and results in a surface layer that is up to 10 times harder than the original material. Ramén Valves began testing the SuperExpanite® treatment in 2019 with excellent results and has decided to adopt it as its standard hardening treatment. This decision not only ensures compliance with the new regulations but also reflects Ramén Valves’ commitment to sustainability and responsible manufacturing practices.
“We believe that by eliminating the use of hard chroming and adopting a pure gas-based process like the SuperExpanite® treatment, we can reduce our environmental impact and contribute to a safer, healthier future for everyone,” says Stéphane Blanchet, Sales Manager, Ramén Valves.
Innovation and sustainability go hand in hand at Ramén Valves
Ramén Valves has a long history of innovation and excellence in the valve industry, and this latest decision reinforces its commitment to provide high-quality, sustainable solutions to its customers. With the adoption of the SuperExpanite® treatment as its standard hardening treatment, Ramén Valves is poised to continue its leadership in the industry.
About Ramén Valves
Ramén Valves is a leading manufacturer of industrial valves for the process industry, including pulp and paper, power generation, mining, and chemical processing. With a focus on innovation, quality, and sustainability, Ramén Valves has established itself as a trusted partner for its customers worldwide.
Expanite, the Danish pioneer and growth company within hardening and heat treatment of stainless steel and titanium, receives its largest capital increase to date. Approximately 12 years after the company was created out of a research project at the Technical University of Denmark, having built a solid business in Europe and having established a strong foothold in the US and Asia, the company will further expand its activities in the US as well as strengthen its presence and offerings in Europe and Asia.
The technology of the company is based on a series of patented processes for high performance, gas phase based hardening/heat treatment of stainless steel and titanium with a reduced environmental impact. Expanite offers a fully digitalized approach to surface hardening, production management and quality control, with a business model that involves a chain of its own service centers as well as technology licensees.
In addition to being able to deliver services that significantly extend the life of components – offering sustainability in its purest form – Expanite’s way to achieve the extended lifetime of materials is the green alternative to environmentally challenged processes such as hard chrome plating and standard stainless steel hardening involving aggressive chemicals and long process times with associated high energy consumption. Expanite’s patented solutions are gas-based, shortening processing time, reducing the overall energy consumption, and eliminating the need for harsh chemical cleaning before/after processing.
Expanite’s CEO Thomas Abel Sandholdt adds: “We are very proud and humble by the sign of confidence that lies behind the biggest strengthening of Expanite’s capital to date. At times when the clouds over the world economy are darker than seen in a long time, Lauritzen Fonden and Finindus have shown that they believe in Expanite’s potential and support an accelerated growth. With this capital increase, we can realize a very ambitious growth plan that involves geographical and product expansions.”
MAN Diesel & Turbo’s newest generation of gas driven engines already plays a key role in the sector’s fuel conversion from oil to alternative energy. When developing the engine, Expanite and MAN Diesel & Turbo collaborated in solving the installation of their gas injection valve in the engine. An important part of the solution came from Expanite – a surface hardening technology that makes stainless steel extremely wear and corrosion resistant.
Per Brandt, manager at MAN Diesel & Turbo, said: “Originally, we had specified a chrome plating in our cylinder cap for mounting our gas injection valve, but it could not always stay tight as there were high requirements for chrome surface quality. We tried unsuccessfully with different solutions. Fortunately, someone had heard about Expanite’s surface hardening, and although stainless steel is usually softer than chrome surface, it turned out to be the solution. A design change coupled with Expanite’s hardening technology solved our challenge.
“In cooperation with Expanite we relatively quickly found a hardening that made our surface resistant against scratches and corrosion. The solution passed our test in service where the system remained sealed and tight and in addition the solution was also easy and cost efficient to implement.”
A cheaper and greener solution solves challenges
Expanite is a Danish growth company established in 2010 by three experts in materials and surface hardening from the Technical University of Denmark. In short, the technology is a gas-based treatment of the surface of stainless steel or titanium, so that corrosion resistance is improved and the hardness is multiplied (eg up to 1200HV 0.05 in stainless steel). This eliminates wear and galling, and the lifespan of the parts is increased by a factor of ten.
“Expanite’s processes are significantly more environmentally friendly and sustainable as opposed to chrome baths”, Claus Løndal, European sales director at Expanite, emphasised. “With Expanite, we have the opportunity to change the rules and extend the limits for what qualities you can expect from stainless steel. In addition, our solutions have the advantage that the equipment and processes can be easily implemented directly by our customers.”
It is expected to be a huge international market for Expanite, whose technology is protected by patents. Expanite has already established service centres in Denmark, Germany, the USA and China, and customers are today coming from the automotive, medical, food and pumps and valves industries, but the demand for more resistant and durable stainless steel products is wide also in the marine sector. “We have had a very positive beginning with Expanite, and as the equipment is to deliver more and more and is pushed to the limits, the need for extra hardening of vital components will also increase”, Per Brandt added.
The German MAN Diesel & Turbo has roots dating back to the 18th century and is today the world’s leading supplier of large diesel engines for marine and stationary plants. The two-stroke engines department is headquartered in Copenhagen, which designs engines and propulsion solutions under the brand MAN B&W
Expanite is based on uninterrupted research since 2000 and was founded in 2010 by leading experts in materials and surface hardening. The company is headquartered in Hillerød near Copenhagen and has hardening capacity in the USA, Germany and China. In addition, Expanite’s solutions are flexible and can be tailored to be introduced into a customer’s own production line as part of a licensing arrangement.
(Articolo in inglese)
The results of a long-term corrosion test show in particular the superiority of the Expanite technology over traditional methods. A horror for every assembler: galling of stainless steel fittings. A drop of lubricant on the thread helps, but what if, due to for example hygienic requirements, no lubricants may be used, or it is simply forgotten?
Stainless steel, with its excellent corrosion resistance, has the disadvantage that when having steel-to-steel contact, it tends to gall. Applying coatings such as DLC or diff usion processes such as Kolsterising® carburizing, nitrocarburizing or plasma nitriding have proven eff ective in counteracting this effect. However, these processes are not only very costly, but also generally have a negative effect on corrosion resistance.
Improved galling and corrosion resistance
With its processes specially developed for stainless steel, Expanite has overcome this dilemma, using its patented technology to improve corrosion resistance of components during hardening and at the same time make them resistant to wear and galling. In this context, TCW Anlagenbau, located in Germany, recently carried out extensive long-term corrosion tests on different stainless steel grades.
While the traditionally hardened components made of 316Ti (1.4571) seen in Figure 1, Figure 2 and Figure 3, show a comparatively strong corrosion attack in the salt bath after a short time, the fittings 316Ti (1.4571) and cutting rings 316Ti (1.4571 and 1.4462) treated with the SuperExpanite show no corrosion even after two months in 10% salt bath solution.
In the case of 303 (1.4305), which is basically less resistant, a tendency towards improvement in corrosion resistance could be demonstrated as the unhardened nozzle (Figure 4) shows a significantly stronger corrosion attack than the component treated with the Expanite process (Figure 5).
Sustainable Against Wear & Galling
“Galling of stainless steel components is a well-known problem and we regularly hear from our customers about cases where the galling of a classic screw connection means that entire component have to be replaced at great expense,” explains Dr. Holger Selg, Expanite Sales Director DACH and application expert. “Ideally, both friction parts are treated with the SuperExpanite, but hardening only one component already provides a significant improvement in terms of galling resistance.”
The relatively low hardness of stainless steel also makes corresponding components susceptible to abrasive wear. Here too, the surface hardening of Expanite provides a remedy and extends the service life compared to unhardened stainless steel components. “Sustainability is playing an increasingly important role,” says Dr. Selg. “Expanite also scores points over comparable wear protection processes with a significantly more energy-efficient and environmentally friendly gas-based diffusion process that does not require time-consuming cleaning of the parts, e.g., with aggressive chemicals after hardening.”
Broad Spectrum of Materials
Expanite technology, specially developed for corrosion-resistant materials, offers a solution for austenitic, martensitic, ferritic and duplex stainless steels as well as for nickel-based alloys like Inconel® and Hastelloy®. ExpaniteHard-Ti, developed by the Expanite team to market maturity in recent years, is used for titanium grades
Hydrogen – whether applied in shipping, steel production, refineries or power-to-x application – it has become the energy source of the future, yet already used in many sectors. Due to its special properties, the hydrogen gas poses several technical challenges for the respective components with which it comes into contact, so that the use of corrosion-resistant stainless steel is essential. This in turn is susceptible to wear and galling and this is exactly where the technology of the Danish pioneer for surface hardening of stainless steel and titanium comes into play.
Hydrogen – the element that comes first in the periodic table and is therefore the smallest and lightest atom among all elements – has been traded for some time as the greatest hope for future energy supply. With a mass fraction of about 70%, hydrogen is the most common chemical element in the universe. On Earth, however, its mass fraction is only 0.87% and is largely bound in water.
Starting with production, over to transport ending with the actual usage, wherever hydrogen is involved, stainless steel components are also used, some of which are subject to extreme wear depending on the application and naturally subject to galling. In the past, this problem was countered with traditional wear protection methods and the respective components were protected with a coating (e.g. hard chrome plating, DLC or chrome carbide) or classic plasma nitriding or salt bath diffusion layers. However, these processes are generally known to affect the corrosion resistance of the stainless steel parts negatively.
The solution to this dilemma – a surface hardening that protects against wear and galling while at the same time maintains or even improves the corrosion resistance of the materials – was developed by the founders of the Danish company Expanite and is increasingly being used in the hydrogen sector. For example, components of high pressure and control valves or clutch systems used in the hydrogen infrastructure up to components for fuel cells are hardened. Sustainably prevent hydrogen embrittlement
Improves wear resistance on stainless steel components
“Our customers often ask whether the Expanite technology increases the risk of hydrogen embrittlement, which is not the case,” explains Dr. Holger Selg, application expert and Expanite Sales Director DACH, who going further into detail: “The cause of the hydrogen embrittlement can be traced back to the diffusion of the hydrogen into the material, and in theory the Expanite layer acts as a diffusion brake to a certain extent. Charged with nitrogen and carbon, the Expanite surface is harder for the hydrogen to penetrate than an untreated surface.”
Expanite – The technology of the Danish pioneer for surface hardening of stainless steel and titanium is
used more and more frequently when it comes to alternatives to classic fluoroplastic coatings such as Teflon®, Halar® or Xylan®. In addition to technical and commercial advantages, the environmental compatibility and sustainability of the Expanite process are playing an increasingly important role for customers.
After the discovery of PTFE (polytetrafluoroethylene) – also known as Teflon® – in 1938, some time passed before technically usable applications for the highly inert material paved their way. Today, coatings based on per- and polyfluoroalkyl substances (PFASs), known by their trade names such as Teflon®, Halar® or Xylan® or more generally referred to by the abbreviations ECTFE, PFA, FEP, ETFE, PVDF etc., are applied in many areas to optimize the sliding and non-stick properties as well as wear and corrosion resistance of components
In addition to the well-known disadvantages of coatings, i.e. inhomogeneous layer thicknesses, layer detachment and increased corrosion attack if the coating is damaged, PFAS-based products have another substantial deficit: the highly stable and therefore not completely degradable carbon-fluorine compounds are harmful not only for humans but also to the environment and can cause cancer, among other things.
Accordingly, there has been a trend towards avoiding the use of these materials since the early 2000s, and especially when it comes to improving wear and corrosion resistance, Expanite surface hardening offers a cost-effective and sustainable alternative.
The Expanite technology is based on a gas process in which carbon and nitrogen are diffused in the component surface. The otherwise comparable soft and wear-prone stainless steel material hardens by a factor of up to 10 and while classic diffusion processes impacting corrosion resistance of the material, the Expanite processes generally improve the corrosion properties of the component.
Sustainable against wear and galling
Besides the elimination of carcinogenic substances, Expanite’s gas-based diffusion process is significantly more energy-efficient and environmentally friendly than comparable wear protection processes, and thanks to the treatment in a vacuum, no time-consuming cleaning of the parts, e.g., with aggressive chemicals, is necessary after hardening.
“The different cases in which Expanite is used as an alternative to such plastic coatings are widely spread and can be found, for example, in the areas of pumps and valves, food & beverage or general industrial applications,” explains Dr. Holger Selg, application expert and Expanite Sales Director DACH, and continues: “First and foremost, we increase the wear and galling resistance of stainless steel components. By hardening the surface, the sliding properties are improved at the same time and, depending on the application, offer an adequate replacement for fluoroplastic coatings.”
The Expanite technology, specially developed for corrosion-resistant materials, offers a solution for austenitic, martensitic, ferritic, and duplex stainless steels as well as for expensive nickel-based alloys such as Inconel and Hastelloy. Moreover, ExpaniteHard-Ti, which the Expanite team of experts has developed to market maturity in recent years, is used for titanium grades.
