|ASTM Adopts New Standard for Hot-Dip Galvanizing of Fasteners
Committee F16 Marks a Milestone with the Publication of F 2329
ASTM Committee F16 on Fasteners has recently adopted a new standard,
F 2329, Specification for Zinc Coating, Hot-Dip, Requirements for Application to Carbon and Alloy Steel Bolts, Screws, Washers, Nuts, and Special Threaded Fasteners. As the name suggests, this standard addresses the coating of threaded fasteners and washers by hot-dip zinc galvanizing. The document was constructed in a manner to complement but not to replace ASTM A 153/A 153M, Specification for Zinc Coating (Hot-Dip) on Iron and Steel Hardware. ASTM F 2329 relies on ASTM A 153 to address the processing parameters and general requirements of hot-dip galvanizing, and the new standard addresses concerns and requirements that are specific to the coating of threaded fasteners. This standard is an important milestone in the path embarked upon by Committee F16 to assume and maintain jurisdiction over coating standards that specifically address the concerns of the fastener industry.
Historically, fastener coatings have been relegated to a subset within general coating standards developed and maintained by such ASTM committees as B08 on Metallic and Inorganic Coatings or A05 on Metallic-Coated Iron and Steel Products. In many instances, important topics such as thread fit, thread tolerances and coating allowance have been dealt with incompletely.
In response to this situation, an important paradigm shift has been occurring within Committee F16 on Fasteners in the course of the last 10 years. The committee has endeavored to develop coating standards that apply specifically to fasteners. At the heart of this effort is the motivation to have jurisdiction and oversight over the standards for the coating of fasteners because Committees such as B08 or A05 may have different priorities from fastener manufacturers, distributors and end users. Requirements that are appropriate for stampings, machined parts and large components such as landing gear are not always applicable to threaded fasteners, especially when one considers their application as clamping devices to which stress is applied in the form of a preload.
There have been several stages to this evolution within Committee F16. At first there was the need to develop standards for a number of organic dip spin coatings, such as Dacromet or Magniguard, that were being developed for automotive fastener applications. North American manufacturers had already embarked on the writing of proprietary standards for these coatings, and it was only a matter of time before nonproprietary standards would be needed. Because the user community would primarily be the fastener industry, the task of creating these standards was undertaken by Committee F16, and more specifically by Subcommittee F16.03 on Coatings on Fasteners. Up to this point the coating standards being developed by Subcommittee F16.03 did not exist elsewhere within the body of ASTM standards.
An ongoing challenge for Committee F16 was the need to have sufficient input into coating standards that existed under the jurisdiction of other committees. Yet, despite a willingness to accommodate the need of the fastener community, the subject of fastener coatings remained outside the scope and core expertise of these committees. Consequently, key topics such as thread fit and coating allowance were not adequately addressed in their standards. Another gap in existing standards was the lack of distinction being made between inch and metric threads. In a peculiar twist that is particular to fasteners, the failed attempt to convert to the metric system in the United States has resulted in inch and metric screw threads being used simultaneously in North America and also around the world. It is not possible to perform a soft conversion between inch and metric thread dimensions because these are separate and different standards with distinct functional and mechanical characteristics. Committee F16 treats the two thread series in separate inch and metric versions of the same standard. Conceptually this practice runs contrary to ASTM’s efforts to convert all of its standards to include both inch and metric units. However, this approach is not possible in fastener standards.
Another area of divergence between Committee F16 and other committees has been on the subject of hydrogen embrittlement. The phenomenon of hydrogen embrittlement is very complex and requires further research before it will be completely understood. Consequently, there is some disparity in the manner in which different standards and industries deal with the question of prevention. Preventive processing requirements such as post baking and test methods stipulated in B08 standards were sometimes too broad or simply not directly applicable to fasteners. One example of this can be found in a document that was widely used in the fastener industry, ASTM B 633, Specification for Electrodeposited Coatings of Zinc on Iron and Steel. Historically, this standard has been the source of considerable controversy between committees F16 and B08 over the subject of hydrogen embrittlement prevention.
Over time it became clear that the expertise and also the drive to address fastener specific needs naturally resided within F16, and the committee was best equipped to handle topics such as standard thread fit and coating allowance criteria.
In response to this growing need, Subcommittee F16.03 began work on developing standards for the most widely used coatings in the fastener industry. In doing so, the subcommittee has endeavored to develop documents that reference pre-existing standards for coating process parameters while spelling out fastener specific requirements such as special processing of threaded parts, coating tolerances, and hydrogen embrittlement prevention. The first of these standards were ASTM F 1941, Specification for Electrodeposited Coatings on Threaded Fasteners, and its metric corollary ASTM F 1941M, which were adopted in 1999 and 2000, respectively. ASTM F 2329 is the latest coating standard resulting from this evolution. Table 1 lists the standards developed by and under the jurisdiction of Subcommittee F16.03.
ASTM F 2329 Highlights and Discussion
Scope and Structure
ASTM F 2329 addresses the requirements for hot-dip zinc coating applied to plain carbon and alloy steel bolts, screws, washers, nuts, and special threaded fasteners. Nails and rivets are not included. Additionally, this specification is intended for fasteners that are handled as batches in galvanizing baskets and spun to remove excess galvanizing batch metal. Spinning or centrifuging ensures that there is minimal accumulation of free zinc in the threads that might hinder thread fit.
In comparison, ASTM A 153 is a hot-dip galvanizing standard applicable to steel hardware of all shapes and sizes. In most cases large articles are galvanized individually by means of a hoist and often cannot or need not be centrifuged. It follows that processing methodology and controls can vary a great deal depending on what is being galvanized. ASTM F 2329 addresses these topics specifically for threaded fasteners and washers, but references ASTM A 153 quite extensively with respect to general galvanizing process guidelines and parameters. The standard carefully avoids stipulating competing or conflicting requirements. Not only does this approach avoid the duplication of standards, it also ensures that ASTM A 153 will remain the repository of the hot-dip galvanizing expertise that resides within both Committees A05 and F16.
It is worth noting that the International Organization for Standardization (ISO) version of this standard, ISO 10684, Fasteners hot-dip galvanized coatings, combines processing guidelines and fastener specific requirements, namely thread tolerance information, in the same document.
Section 3 defers to ASTM F 1789, Terminology for F16 Mechanical Fasteners, for its fastener terminology while providing a number of definitions that are specific to hot-dip galvanizing of fasteners. Notable are the definitions of “high temperature galvanizing,” which is performed at a higher temperature than conventional hot-dip galvanizing, and the definition of “production lot” in the context of the batch processing of fasteners in galvanizing baskets.
Section 5.2.1 introduces a batch lot level sampling plan for processes based on a prevention control plan as opposed to a detection control plan. Section 10, in conjunction with the standard’s Tables 1 and 2, details the sampling plan for key characteristics based on either prevention control or detection control. This approach is in accordance with ASTM F 1470, Guide for Fastener Sampling for Specified Mechanical Properties and Performance Inspection.
Physical Alteration of the Coated Fasteners
Section 5.4 strictly prohibits any unauthorized physical alteration of the parts after galvanizing by a machining process. The primary reason for this restriction is to prevent any uncontrolled influence on the mechanical properties of the fastener, and to ensure that thread dimensions remain within the original specifications. A secondary consideration is to prevent unintended removal of the coating from any part of the fastener, as this will compromise corrosion protection.
Secondary Coating Processes
Section 5.5 allows secondary coating processes at the discretion of the purchaser. Examples of this are the application of post chromate for additional corrosion protection of the zinc coating itself, phosphating to improve paint adherence or the application of a lubricant for ease of assembly.
Effect of Temperature
Section 7.2.1 provides guidelines for preventing the alteration of mechanical properties of the fasteners by exposure to high temperatures. The fastener supplier must ensure that the galvanizing temperature does not exceed the tempering temperature from heat treatment. This concern usually occurs when the galvanizing temperature exceeds 800°F.
Section 7.2.3 prohibits the galvanizing of fasteners that exceed 1 inch in diameter with a specified minimum hardness of 33 HRC. This restriction is designed to safeguard against microcracks that may occur as a result of the thermal gradient that can occur within large diameter fasteners during galvanizing.
Section 8.2 makes provision for the physical distortion of parts by exposure to heat, and spells out that it is the responsibility of the purchaser to ensure that the parts are capable of withstanding exposure to the heat of galvanizing. Such a provision does not exist in ASTM A 153.
Hydrogen Embrittlement Prevention and Testing
Section 7.2.2 warns of the risk of internal hydrogen embrittlement when parts with a specified hardness of 33 HRC and higher are acid pickled prior to galvanizing. The section does not go so far as to mandate the use of mechanical descaling instead of acid pickling or baking before galvanizing; it defers to the prevention method specified in the product standard or by the purchaser.
Section 11 stipulates that when required, hydrogen embrittlement testing shall be performed according to ASTM F 606, Test Methods for Determining the Mechanical Properties of Externally and Internally Threaded Fasteners, Washers and Rivets. In comparison, ASTM A 153 only deals with the subject of hydrogen embrittlement in very general terms and does not make any provision for prevention or testing.
Section 5.4.2 specifies that dimensional allowance must be made to accommodate the coating thickness on externally threaded fasteners to ensure proper fit and functionality.
Section 8.1 represents another important enhancement as it stipulates the use of “coating thickness” as the product control characteristic versus the “coating weight” used in ASTM A 153. Measuring coating weight is a lengthy and sometimes problematic process. For example, threads constitute an additional complication when calculating the coating surface and therefore true coating weight. On the other hand, measuring coating thickness by means of magnetic or eddy-current devices is quicker and a great deal more practical. The practicability of measuring coating thickness also makes it a more effective process and product control tool.
Section 11.2 specifies a minimum number of readings required to obtain an average coating thickness, and it also designates a referee method in case of disagreement. The referee method consists of an optical measurement of coating thickness on a cross section of the part in accordance with test method ASTM B 487, Test Method for Measurement of Metal and Oxide Coating Thickness by Microscopical Examination of a Cross Section. ASTM A 153 does not specify a referee method for measuring coating thickness.
Although washers are not threaded fasteners, they are often used with threaded fasteners, and are therefore included in ASTM F 2329. Section 9 contains a note that addresses the galvanizing of washers. Due to their flat geometry, washers tend to stick to one another during hot-dip galvanizing. This situation has two immediate consequences. The first is the incomplete coating of the washer, which can compromise corrosion protection. The second is that it may result in customer dissatisfaction unless acceptance criteria are agreed upon between purchaser and galvanizer. In the absence of a prior agreement, the galvanizer must ensure that the washers are not bonded to one another by adapting the parts handling mechanism in a manner that is best suited for the processing of washers.
The adoption of F 2329 is a significant success for ASTM Committee F16 on Fasteners. It is important to note that a key ingredient to this success has been the collaboration of Committee A05 in creating a document that complements ASTM A 153. Potential users of F 2329, which include fastener manufacturers, batch galvanizers, distributors and end users, are encouraged to use this document because it is a more appropriate and applicable standard for galvanizing threaded fasteners. Not only will ASTM F 2329 be a valuable resource for its users, it also will inevitably lead to improved products by addressing the elements that are critical to the fastener community. Of equal significance is that the vibrant and open committee structure within ASTM will ensure the continuous improvement and evolution of this document in the course of years to come. //