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Galling and Its Control AUSTRALIAN STAINLESS STEEL DEVELOPMENT ASSOCIATION GALLING AND ITS CONTROL ASSDA Technical FAQ No 5 5 WHAT IS GALLING? THE DETAILS Goodness of fit Austenitic stainless steels are widely used Surface›hardness Parts should be dimensionally tight for corrosion resistant bolting. One of the enough to prevent vibration and wear Surfaces treated so that they are harder, major problems in use is that disassembly with consequent roughness which might for example, hard chrome plated, nitrided, is difficult because nuts and bolts seize. facilitate galling. However, sufficient carburised or cold worked surfaces are This phenomenon is known as galling and clearance is required to avoid fouling usually less susceptible to galling. it is most prevalent with intermittently during assembly. operated, slowly sliding surfaces. It is However, the corrosion resistance of Quality Control caused by cold welding of the high points nitrided or carburised surfaces is less than of clean, oxide free metal left when the standard pickled and passivated stainless Good housekeeping is needed to exclude oxide film is dislodged by surfaces rubbing steel. Hard chromium electroplated dirt or abrasive materials from between against each other. In high temperature coatings (3µm to 0.5mm) are not mating surfaces. It is also vital that the service, there may also be a contribution recommended for high temperature or thread profiles match, ie. diameter, from oxidation forming abrasive oxides and high stress service as they may crack and clearance and thread form. also reducing clearances. Diffusion bonding spall. of the metal is also possible during long A shot peened (and hence cold worked) Reduce›friction term exposure but is not as likely to cause surface would not normally be suitable. Lubrication problems at reasonable temperatures. It will be rougher and thus be more There are a wide range of lubricants Titanium and aluminium also suffer from susceptible to galling because of the containing nickel, molybdenum, copper, galling. Specialist literature suggests that roughness despite the increased hardness silver or graphite or combinations of anodising or lubrication are both effective induced by the compressive stress from the these. They can be messy. Those primarily remedies. Anodising of stainless steel, peening. Cold rolled surfaces will be harder containing nickel are more suitable for high however, is not regarded as a remedy and usually more galling resistant than temperatures. Two suppliers of nickel and since the chromium oxide film is orders of machined surfaces. graphite containing anti-seize compounds magnitude thinner than the oxide film on There is a rule of thumb that a hardness are Devcon and Loctite. Their websites titanium or aluminium. difference of 50 Brinell is required for offer selection tables depending on service effective prevention of galling and this is requirements. Unasco also offers both a HOW TO CONTROL GALLING illustrated in the experimental data shown nickel and a silver impregnated Teflon® on the next page. tape. Molybdenum disulphide may also be The measures listed are roughly in order of suitable. importance and are explained in the next Design›and›quality›control section. The graph below shows that using a Surface finish ››hardness: ensure a hardness lubricant makes a substantial difference to difference of at least 50 Brinell Highly polished surfaces (Ra<0.25µm) or the torque value for the bolt condition. between the nut and bolt. very rough surfaces (Ra>1.5µm) tend to gall ››design ›and›quality›control: confirm more. Cold rolled surfaces are better as that the mechanical fit is correct, they tend to be smoother than machined the surfaces are smooth but not surfaces. Machined threads should be mirror like and that they are clean. carefully deburred before assembly. Inspection with a low power lens (x5 or ››reduce ›friction: select an appropriate x10) is a useful control method or, as a lubricant and/or substrate material. practical test, if a cloth snags when wiped Lubrication is required even if the over the thread surface, then the bolt is joint is intended to be permanent. likely to gall when assembled. Significant ››control: use a torque wrench and galling problems may also arise with the correct force for the material electropolished surfaces which can be class and surface conditions. very smooth, ie. Ra<0.25µm. However, electropolishing smooths rough edges and for this reason is an advantage in reducing galling tendency. Be aware of possible crevice corrosion is used, then galling could be minimised can lead to fatigue or even permit the nut between the grease and the steel if the by using a hard nut of duplex stainless to shake loose. grease dries out and cracks or if the grease steel or martensitic 431 stainless steel. The torque table below was calculated does not properly wet the surface so that a Alternatively, a soft (and also less corrosion (www.bufab.com) by assuming clean, cavity forms allowing water to accumulate. resistant) nut of aluminium bronze could burr-free, flat surfaces with a good quality If graphite is used in large quantities then be used, provided that the environment lubricant - with a friction coefficient of galvanic corrosion of the stainless steel is was not so corrosive that it caused rapid 0.16. These values are only a guide and in possible. Nickel plating has been used as wastage of the aluminium bronze nut. critical applications, trials may be required. a protection against such corrosion. Silver The class ratings are from ISO 3506 and plating has also been used as a lubricant Surface›contact›stress apply for A2 (304) and A4 (316) fasteners. to prevent galling of nickel-copper (Monel) alloys in oxygen service. Reducing the surface pressure is effective in reducing the risk of galling. This may be WHAT IS EXPERIMENTAL Material Selection achieved by increasing the contact area EVIDENCE? If the system is not lubricated or has or reducing the load, so that there is less The table in the centre column shows insufficient lubrication, proper grade stress on parts in contact and less depth the results of rotating pin-on-block selection is vital to limit galling. There can of wear. Over-tightening the bolt will give experiments for several material pairs be an improvement obtained by using excessive surface stresses and the only way (ASTM G98). It indicates the threshold two different stainless steels across the to control this is to use a torque wrench stress where galling commences for interface. This is largely related to the and torque appropriate to the grade of different materials in contact with 304 magnitude of the hardness difference stainless steel, size, pitch and the state of grade stainless steel using a button and between the two steels and some lubrication. rotating block test. The hardness of the experimental information is shown Under-tightening is also a risk if the bolt is steel in Brinell hardness values is given for opposite. There are also grades which are not clean and, if there are cyclic loads, this each material. self-lubricating such as the high nickel grade Waukesha 88 which contains tin and MEASURED GALLING STRESS CALCULATED FASTENING TORQUE bismuth or Waukesha 54C which contains TORQUE - NEWTON.METRE (Nm) tin and silver. The sulphide particles in free METAL A METAL B THRESHOLD (BRINELL (BRINELL STRESS DIAMETER PITCH CLASS CLASS CLASS machining 303 also appears to provide HARDNESS) HARDNESS) (MPa) mm (D) mm 50 70 80 lubrication. The corrosion resistance of these grades should be carefully S30400 S30400 13 M3 0.5 0.4 0.9 1.2 (140HB) (Austenitic, considered. In addition, some lower annealed - 140HB) M4 0.7 1 2 2.7 nickel, 200 series grades with nitrogen, M5 0.8 2 4 5.4 manganese and silicon additions, such S31600 13 (Austenitic, as ARMCO’s Nitronic® 60 or Carpenter annealed - 150HB) M6 1.0 3 7 9 Technology’s Gall-Tough® and Gall-Tough S4300 (Ferritic, 13 M8 1.25 8 17 22 Plus®, strongly resist galling. annealed - 159HB) M10 1.5 15 33 44 It should be noted that while using 304 S41600 165 M12 1.75 27 57 76 bolts with 316 nuts is often quoted as a (Martensitic, hardened & stress cure for galling, the hardness difference is M14 2.0 43 91 121 relieved, sulphur not necessarily sufficient to prevent galling added, 342HB) M16 2.0 65 140 187 even with a cold rolled, harder 304 thread used against an annealed, softer 316 nut. S41000 >340 M18 2.5 91 195 261 (Martensitic, There are reports that A2-80 and A4-80 hardened & stress M20 2.5 127 273 364 bolts are more resistant to galling than relieved, 352HB) 70 class bolts but there are other reports M24 3.0 220 472 629 S21800 (Nitronic >340 (www.cartech.com “Galling and stainless 60 - Austenitic, M27 3.0 318 682 909 steels”) that cold work increases galling annealed - 205HB) risk. M30 3.5 434 930 1240 Waukesha 88 >340+ M36 4.0 755 1620 2160 If an austenitic bolt such as 304 or 316 (141HB) (contains Sn and Bi) Elongation when tightened 0.6D 0.4D 0.3D The technical recommendations contained in this publication are necessarily of a general nature and should not be relied on for specific applications without first securing competent advice. Whilst ASSDA has taken all reasonable steps to ensure the information contained herein is accurate and current, ASSDA does not warrant the accuracy or completeness of the information and does not accept liability for errors or omissions. Level 9, 307 Queen Street T +61 7 3220 0722 Brisbane QLD AUSTRALIA 4000 E [email protected] www.assda.asn.au networking››promotion››education››accreditation››advocacy››problem›solving.
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