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Surface-engineeringfor selecting the best finish.SOLUTIONS with Effective, Powerful Advice
Surface-engineering was the subject of one of our Mid Month Bulletins. In that page, as usual, we provided a wealth of original information available online from different sources.
Click on PWL#046B to read it. We were happy to get positive feedback on our effort. An Article, on this topic was published (7) in Issue 34 of Practical Welding Letter for June 2006. It explains selected aspects of the technologies that address issues of surface modification. The purpose of applying Surface-engineering technologies is to enhance determined surface characteristics for improving performance in service conditions. Even those knowing quite a bit on certain aspects of the subject of this page might profit from this exposition. It is probably useful to review if and how improved surface properties might interfere with welding. In general, with the exception of Hardfacing which is itself a welding process (see further down), welding should never be attempted on items modified by Surface-engineering. We hasten to present here an overview on this important subject. Different Surface-engineering modification processes can be used to enhance operation and durability of different items. Unless specific experience is available, selection is based generally on previous knowledge of what is used, on similarity of application, on testing and on known results.
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The importance of the integrity of the external surface of any object derives from the fact that the surface is the first place where interactions with the environment occur. It is quite common, furthermore, that the largest stresses concentrate on the surface. The continuing satisfactory performance of the functions that the item is made to absolve in service depends essentially on the integrity of its surface. If the surface fails locally because of aggressive attacks, destructive processes like wear or corrosion, fatigue or creep can easily progress unhindered, until the item fails to provide the designed functions.
The different processes of Surface-engineering can be grouped under the following headlines:
Surface Modification - Shot Peening These are processes that introduce compressive stresses on the surface of treated elements. Thus modified surfaces provide a much increased resistance to fatigue (cyclic stressing). See our Website page on Shot Peening. An Article on Surface Impact Treatments was published (7) in Issue 42 of Practical Welding Letter for February 2007>. An Article on Ultrasonic Impact Treatment was published (11) Surface Treatments These include Thermal treatments like Case Hardening and other Surface-engineering protective treatments that induce penetration and thermal diffusion of different atoms (Chromium, Aluminum) in steels and nickel alloys. An Article on Low Temperature Carbon Supersaturation was published (7) in Issue 38 of Practical Welding Letter for October 2006. It reports on efforts to develop Surface hardening processes also for austenitic stainless steels, without impairing their corrosion resistance properties. Such processes are applied by commercial companies under proprietary names. See the following document: Coating This is a general name that does not identify the process used. Surface-engineering drawing requirements must establish what has to be done. It means covering a material surface with a layer of another substance. The purpose is to provide to the treated surface some special characteristics that are not present in the original stuff. An Article on Oxidation Resistance and on coatings suitable to provide protection to retard it, was published (2) in Issue 56 of Practical Welding Letter for April 2008. An Article on Nanocomposite Coatings was published (11) in Issue 142 of Practical Welding Letter for June 2014.
Some confusion may exist in the definitions so that specific processes are not always listed in the same category.
Coating of steel strip by immersion in molten metals like tin, zinc, lead or aluminum are relatively old processes. They were used for quite a long time to make steel for cans, usable for preserving food, or to protect from corrosion (rust) exposed surfaces of steel items. Within the coating category one should include also non metallic coating processes, like phosphate and chromate coating, porcelain enameling, ceramic coating and painting. A special case of the above is called Vacuum coating It provides a solid object with a thin deposit of coating material that is vaporized in a vacuum chamber and made to condense onto the substrate.
Physical Vapor Deposition (PVD) and Chemical Vapor Deposition (CVD) are coating processes used for special applications.
Sputtering and ion implantation are modifications of the process that change microstructure and properties on the substrate surface. Thermal spraying is sometimes included in the coating category: it should be noted that different processes come under this title. See also our page on Thermal Spray.
Despite the ambiguity deriving from the name, indicating a low temperature process, also Cold Spray is generally included in Thermal Spray.
Hardfacing are processes that involve welding. These are usually grouped within the Coatings category of Surface-engineering as they apply a different metal on top of the material of which one wants to improve the surface properties.
An Article on Filler Metal for Ni-WC Hardfacing was published (4) on Issue 161 of Practical Welding Letter for January 2017. See our Hardfacing page. Cladding of a more noble material onto a common metal substrate can be performed by explosive welding, by roll bonding or more recently by friction stir welding. The common metal provides the required strength, the clad material gives the resistance to corrosion or heat or both. Welding of other items to clad material is sometimes possible with utmost care. Plating is one of many Surface-engineering processes used for depositing a layer of metal atoms upon a different conductive metal in a way that guarantees adhesion and provides hardness or corrosion protection. In electroplating the process uses an electric current to deposit metallic ions from an electrolytic solution in thin layers on the conductive surface that has to be plated. The anode, that includes the metal to be deposited, is connected to the positive terminal of the direct current supply. The cathode, that includes the article that is to be plated, is connected to the negative terminal. A special case is the conversion of the surface of a metal to its oxide, called Anodizing, an electrolytic process. It is typically applied on aluminum or magnesium. The product is a relatively thick oxide layer adherent to the metal. A small increase in dimensions has to be taken into account. A variant of the process provides Hard Anodizing, suitable for wear uses but possibly sensitive to impact or fatigue applications.
Another special is Electroless plating, also called auto-catalytic plating. It is performed without the use of electric power, especially for deposition of nickel. Plating processes are severely scrutinized by authorities for their possible negative impact on the environment. Metal Finishing is a general category of Surface-engineering processes used to remove burrs and asperities deriving from previous machining operations and to improve the smoothness of the final surface. It is not only an aesthetic requirement, because surface irregularities can promote stress concentration and initiation of fatigue cracks. Therefore such delicate locations as holes in gas turbine discs are finished under severe control operations. These are called sometimes super-finishing operations and are examined with utmost care to make sure that no dangerous spots (likely to initiate a crack) are present on their surface. In certain products such as stainless steel sheets the level of finishing is indicated with a known designation to specify the required appearance.
A list of common finishes can be found in the commercial page:
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An Article on Minimizing Alpha Case in Titanium Surfaces was published (11) in Issue 152 of Practical Welding Letter for April 2016.
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To reach a Guide to the collection of the most important Articles from Past Issues of Practical Welding Letter, click on Welding Topics.
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