Fastener Metallography: Assuring the Highest Quality of Compliance

U.S. Fastener Manufacturers experienced tough times in the 1980s with a substantial loss of business because of aggressive competition from offshore suppliers, resulting in massive downsizing and plant closings. Fastener users were then shocked when they discovered that a number of fasteners they purchased did not meet stated properties for their grade. In addition to this, bogus bolts were identified to be the cause of some well documented service failures. This prompted a full blown investigation by the Committee on Oversights and Investigations of the Committee on Energy and Commerce of the U.S. House of Representatives.

Losing business and jobs was a grave problem but dumping counterfeit products using falsified documents stirred Congress into action. Numerous incredible violations that placed the U.S. aerospace, military, space program and the entire manufacturing industry in jeopardy were revealed in the 61 page report published by the committee in 1988. The Fastener Quality Act of 1990, PL 101-542, was passed mandating stringent adherence to established fastener test methods and protocols and tough penalties for violators, domestic or foreign. The original form of the law was found to be unenforceable and after six years of committee meetings President Clinton signed PL 104-113 in March of 1996 (the May 27, 1996 date of compliance has been postponed).

This article illustrates how those who conduct the different metallographic procedures required under the terms of this law can be assured of the highest quality of compliance.

The Role of Metallography

Table 1 presents a list of various possible tests that might be needed by a Manufacturer of fasteners or any other company that alters fasteners for sale. It should be noted that the law applies to specific grade-marked fasteners mainly used or sold in the United States. Distributors are excused from testing as long as the fasteners are not altered before reselling. An article by Joseph Greenslade in the American Fastener Journal, Jan./Feb. 1997, Vol. 14, No. 1, provides further information about the new law. It is also essential to be aware that all tests are not needed by all fastener specifications. Each fastener specification will call out the particular tests that are needed.

Metallographic Specimen Preparation of Fasteners

Sampling/Sectioning

Fastener specifications almost always require that representative fastener specimens be sectioned longitudinally in order to reveal the profile of the head, threads and a minimum length of shank as illustrated in Figure 1. The most efficient way to accomplish this is to cut slightly off-center to allow sufficient material for loss during the subsequent abrasive grinding and polishing. Usually, smaller fasteners are mounted and ground to plane rather than sectioned before mounting.

Table 1. Metallographic Tests for Grade Marked Fasteners.

Flaw Detection or Verification Identification, Location, Extent
Qualitative Analysis Microstructural phases, Grain Flow, Presence of decarburization, Presence of retained austenite, Inclusion identification
Quantitative Analysis Bulk hardness, Microindentation hardness, Decarburization depth, % Retained austenite, Inclusion type, size, distribution, Microdimensions

 

Metallographic cutters and holding chucks are generally designed for general applications and are not best suited to handle the different fastener shapes and sizes. A small accessory chuck is available for securing fasteners and is clamped in the larger cutter vise. A better alternative is the use of a special fastener vise and the ISOMET™ 2000 Precision Cutter, shown in Figure 2. This cutter is uniquely suited for fastener sectioning since it is automatic and provides a high degree of control over the cutting conditions including precise location of the cut to be made. Additionally, using the thinner abrasive wheels designed for this precision cutter, accurate low kerf loss cuts could be made with a surface superior to that produced by standard metallographic cutters.

Scheme for longitudinal sectioning.

Figure 1. Scheme for longitudinal sectioning.

ISOMET™ 2000 Precision Cutter.

Figure 2. ISOMET™ 2000 Precision Cutter.

Mounting Cut Specimens

A correctly sectioned fastener reveals the profiles of the threads in order to discover rejectable defects when examined in the as-polished condition at a low magnification. Different specifications include a drawing, such as Figure 3, that shows the pitch diameter with the most critical defect area being below this line including the thread root that is of key concern to the analyst. Since potentially rejectable defects usually take place at the edges of the threads, it is vital that these edges are not rounded during preparation, making the specimen edge difficult, if not impossible, to precisely analyze. Edge rounding is brought about by wrong choices made in mounting and in the subsequent abrasive surface preparation.

The location of the pitch diameter of the threads.

Figure 3. The location of the pitch diameter of the threads.

Three steps that guarantee maximum edge retention are:

  • Specimens must be thoroughly cleaned in order to remove particles and oily films that prevent the mounting media from mechanically adhering to the surface of the fastener specimen. Ultrasonic cleaning is greatly recommended. Scrubbing the specimens with a fine brush soaked with warm water and detergent is a more labor-intensive alternative. It is then necessary to thoroughly dry the cleaned specimens before mounting.
  • EPOMET® Mounting Resin, an epoxy compression mounting resin that adheres well to a clean surface, can be used to obtain best edge retention. Since it is more expensive than other mounting materials, use EPOMET resin just for protecting the specimen itself and add the softer less expensive phenolic resin as the bulk material as shown in Figure 4.
  • Keep mounts under pressure during cooling in order to prevent the mounting material from pulling away from the fastener specimen because of dissimilarities in the coefficients of expansion and contraction of the resin and the metal, Figures 5a and 5b. The mount should never be removed from the press hot and placed in cold water. A mounting press such as the SIMPLIMET® 2000 shown in Figure 6 which automatically heats and cools the specimen mounts under pressure majorly contributes to the production of shrinkage-free mounts.

Dual resin mounting.

Figure 4. (left) Dual resin mounting.

(top) Shrinkage gap (arrow) in a phenolic resin mounted fastener surface (ejected hot). (bottom) Tight adherence (arrow) of EPOMET® Mounting Resin.

Figure 5a. (top) Shrinkage gap (arrow) in a phenolic resin mounted fastener surface (ejected hot). Figure 5b. (bottom) Tight adherence (arrow) of EPOMET® Mounting Resin.

SIMPLIMET 2000 Automatic Press.

Figure 6. (right) SIMPLIMET 2000 Automatic Press.

Specimen Surface Preparation

Obtaining an accurate analysis of fastener specimens needs both the selection of an efficient preparation procedure and the equipment that develops well prepared specimens economically, rapidly and as flat as possible. The typical procedure presented in Table 2 works for different ferrous fasteners.

The Role of Automation

Automation is not a luxury in the abrasive surface preparation of fasteners but offers the following key benefits:

  • Increased productivity – more specimens per shift
  • Superior results both in finish and flatness, and more uniform, repeatable results
  • Less manual skilled needed – skill dependency eliminated

Table 2. Typical Procedure for Preparing High Quality Polished Fastener.

Step Surface Abrasive Type/ Size Speed (rpm) Time (min.)
Planar Grind CARBIMET® SiC Paper 120 grit 120 Until Plane
Specimen integrity CARBIMET SiC Paper 240 grit 120 1
ULTRA-PAD® Polishing Cloth 9 micron Diamond* 120 2-4
TEXMET® 1000 Polishing Cloth 3 micron Diamond* 120 2
Final Polish MICROCLOTH® Polishing Cloth 0.05 micron Alumina** 120 1

*METADI® SUPREME Diamond Suspensions
**MASTERPREP® Alumina

Automatic specimen preparation machines use either 12” or 8” diameter formats and may be classified as fully or semiautomatic automatic. The PHOENIX® 4000 preparation system shown in Figure 7 will produce exceptional results using either individual load application (one or more specimens) or central force application (full holder).

PHOENIX Specimen Preparation System

Figure 7. PHOENIX Specimen Preparation System.

Results of Quality Fastener Specimen Preparation

It is essential to examine well-prepared fastener specimens in the as-polished condition, preferably after a light etch and brief re-polish in order to remove any residual superficial deformation (smear) that may hide a fine crack. A fine crack in the root of a fastener in the as-polished condition is shown in Figure 8. Observation of the crack could have been masked by flow lines in the root if this specimen would have been etched without examination in the as-polished condition. Figure 9 shows serious defects near the root of this A-286 fastener, clearly shown after light etching. Figure 10 displays a shallow total decarburized layer containing fine oxides in this heat-treated alloy steel fastener at 400x after being originally detected at 100x.

Fine root crack. Mag. 100x; Etchant: None.

Figure 8. Fine root crack. Mag. 100x; Etchant: None.

Near root defects. Mag. 100x; Etchant 50-50 HCI-H2O2 (3%)

Figure 9. Near root defects. Mag. 100x; Etchant 50-50 HCI-H2O2 (3%).

Total decarburization layer at 400x

Figure 10. Total decarburization layer at 400x.

Tech-Tips

Question: How useful are the techniques for edge retention?

Answer: In the days of manual polishing, edge retention was a more complicated problem. The switch from hot ejecting thermosetting type phenolic mounts (which people mostly cooled in water) to cooling under pressure after curing has almost eliminated the shrinkage gap problem, particularly with mounting compounds such as EPOMET® compression mounting epoxy.

Further, use of the newer “hard” cloths and automatic polishing devices make relief relatively easy to control. People used canvas and other “softer” cloths, years ago, including some with a nap, which inherently create relief, regardless of the polishing equipment. Electroless nickel plating does provide the ultimate in edge retention, however it is time consuming. Adding different filler materials (like alumina shot or cast iron) to a mount is not needed today. These filler materials added little benefit and could make etching or polishing extremely difficult.

Question: When epoxy is used, “gassy” mounts full of bubbles are often obtained and sometimes there are problems getting the mount to cure so that it is hard. What mistake has been made in this process?

Answer: First, all liquid resins have a specific shelf life and with time beyond the normal shelf life, curing can indeed become a problem. With any liquid mounting system, it is vital to carefully follow the instructions. Most systems work best when the hardener and resin are weighed to the specified amounts before mixing. This works better than mixing on a volumetric basis, even though it is less convenient. When the epoxy if mixed, it is essential to gently stir the fluids for about a minute.

Strong stirring traps air in the liquid which produces the bubbles, which may not float out based on the viscosity of the epoxy. Some epoxy systems need curing at a temperature other than room temperature, while others just cure faster at high temperatures. However, curing faster may not always be beneficial. It is more difficult to control fast curing epoxies. Generally, elevated exotherm temperatures obtained during curing increase the risk of shrinkage problems. Several users of epoxy mix and fill mounts at the end of the day and remove them from the molds the next morning.

This information has been sourced, reviewed and adapted from materials provided by Buehler.

For more information on this source, please visit Buehler.

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