[ Surface Hardening ]
[ Surface Hardening ]
Improving material performance with surface hardening
Stainless steel screw connections can be susceptible to galling due to factors such as insufficient surface hardness , excessive roughness , or lack of lubricants . Surface hardening offers a solution to this problem .
By Mia Grundtvig , Expanite
Fasteners are essential in many applications to allow for particularly frequent assembly and disassembly or high tightening torques , so it is important to carefully tailor the properties of such elements . When fasteners are jammed due to galling , removal can be an enormous challenge , especially to avoid damaging the bolt or destroying the 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 ( Figure 1 ). 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 .
Figure1 . Generic image of a cold-welded screw connection
can potentially lead to the egg-shell effect . Expanite has developed its proprietary technology ( SuperExpanite ® ) to solve these shortcomings . In this process , not only the outermost layer but also the underlying material is hardened to a greater depth , which also often contributes to an improvement in corrosion resistance . In many cases , a two-stage process is used . 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 300 HV for austenitic materials and 850 HV for alloys with martensitic structure to a depth of up to 1 mm . In the second step , ExpaniteLow-T ( low-temperature process , see Figure 2 ), the workpiece is heated to temperatures below 500 ° C and the surface layer is hardened to 1100-1300HV at a
Figure 2 . Micrograph of expanded austenite layer created with ExpaniteLow-T process ( as a part of SuperExpanite product ) on AISI 316L alloy .
hardening depth of 5-30μm ( depending on the solution ). The hardening is achieved by incorporating large amounts of interstitial carbon and nitrogen . By combining the two process steps , the socalled eggshell effect is avoided , offering a decisive advantage for applications requiring improved load-bearing capacity ( Figure 3 ).
Wear resistance To test wear resistance , 316L samples hardened using its SuperExpanite ® process to ASTM standard G 133 were tested . In this
The surface hardening 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 long been available , their application has two main disadvantages : firstly , all classic processes , such as salt baths , 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
Figure 3 . Hardness profile of AISI 316L . Below the extremely wear-resistant surface layer ( produced in the ExpaniteLow-T process , approx . 1.200 HV ) lies a medium-hard transition layer ( 250-300 HV ), which is created in the ExpaniteHigh-T process . Power curve fitted for visual aid .
26 Stainless Steel World October 2024 www . stainless-steel-world . net