TWO GALLING RESISTANT STAINLESS STEELS USED FOR BRIDGE HINGE PINS

December, 1996

By John H. Magee

Carpenter Technology Corporation
Reading, PA
USA

Introduction
Stainless steels are used by many industries throughout the world for their corrosion resistance, strength, and high temperature characteristics. Recently, they have replaced low alloy steels as hinge pins in long span bridges due to problems with heavy rusting leading to expensive retrofitting or even a bridge collapse.

Bridge pins, 13 to 23 cm (5" to 9") in diameter in lengths of 30 to 46 cm (12" to 18"), act as hinges that move with expansion and contraction of the bridge. These fracture-critical members must absorb the moving load of high volume overhead traffic and resist corrosion due to the heavy use of road salt and the big temperature swings that can occur in rugged northern environments. Thus, bridge pins have to possess high strength, corrosion resistance and galling resistance.

These requirements do create a problem for design engineers who specify materials. Typical stainless steels, like Type 304 or 316, do not possess the galling resistance required for this application and, lubricants are not the solution due to the long life-cycle of a bridge and lack of accessibility of these pins. Thus, alloy selection is critical. Two new galling resistance stainless steels have been developed which uniquely resist galling and excessive wear under heavy loads without lubricants or coatings. These alloys, Gall-Tough® Stainless and Gall- Tough® PLUS Stainless, have been used as hinge pins. This paper will discuss the research performed to develop these alloys.

What Is Galling?
Galling is a severe form of adhesive wear which occurs when two metallic components slide against each other at relatively low speeds. With high loads and poor lubrication, galling can cause surface damage characterized by localized material transferor removal. The damage caused by galling may occur after just a few cycles of movement between the mating surfaces. Severe galling can cause seizure and costly problems.

Mated surfaces typically show distinct weld junctions when galling takes place. These are areas where asperities, or surface protrusions, from one surface have welded with those on the other surface. Under low stresses, the junctions are minute, and break apart with movement. However, higher stresses produce much larger weld junctions and galling. Materials with limited ductility are less prone to galling since even under high loads surface asperities will tend to fracture when interlocked. Small fragments may be lost but the resultant damage will be similar to scoring rather than galling. For highly ductile materials, like Type 304 stainless, asperities tend to plastically deform; thereby, increasing the contact area of mated surfaces and eventually galling occurs1.

Galling Test
In the 1950's, a simple button-on-block test was developed to evaluate the galling resistance of stainless steels. Figure 1 depicts the galling test arrangement showing a button specimen on the block sample. Immediately prior to testing, both galling samples are cleaned to remove machining oils and metal particles. Then a compressive load is placed on the 1 .3 cm diameter (0.5 in.) button resulting in a specific compressive stress. The button is then slowly rotated with a wrench counterclock-wise 360°, clockwise 360° and then counterclock-wise 360°. The compressive load is then removed and the mated surfaces visually examined for galling. If no galling is observed, a new button is tested at a higher compressive stress level.

Threshold galling stress values are determined to within 6.9 MPa (1 ksi), and duplicate samples are generally tested to confirm the highest stress level at which no galling occurred for each alloy. The highest stress level at which galling does not occur is defined as the threshold galling stress (TGS).

Gall-Tough Stainless Development
In order to develop a stainless steel with improved galling resistance, various experimental compositions were induction melted in Carpenter's Research lab and cast as7.0cm (23/4 in.) sq. ingots, weighing 7.7 kg (17 Ibs.). These ingots were forged to bar, annealed (1950F/WQ) and galling specimens machined for testing. The base composition for these experimental stainless steels was 0.10%C, 16%Cr, 0.15%N and balance iron. Critical elements varied were silicon (0.6 to 4.0%), manganese (1.9 to 6.0%) and nickel (2.5 to 10%) contents. The composition of the experimental alloys are listed by increasing silicon content in Table 1.

Figure 2 plots threshold galling stress (TGS) data vs. silicon and nickel + 0.5 manganese for the experimental alloys. A box has been drawn to define the Si and Ni +0.5 Mn ranges for an austenitic stainless steel with improved galling resistance; i.e., having aTGS >62 MPa (>9 ksi). The ranges are as follows: Si = 1.0 to 5.0%,"and Ni + 0.5 Mn = 5.5 min. and (11 x % Si + 42)/8 max. The final ranges selected for Mn and Ni separately were as follows: Mn = 2.0 to 7.0% and Ni = 2.0 to 7.75%.

Galling results determined that silicon was very beneficial to galling resistance, while nickel and manganese were deleterious. Nickel was approximately twice as harmful as manganese. In addition to galling resistance, the Si and Ni + 0.5 Mn contents were defined by phase balance (formation of excessive ferrite or martensite in the austenitic matrix). Thus, a minimum Ni + 0.5 Mn level of 5.5 was required to maintain the desired austenitic structure, especially since stable martensite in the annealed condition is quite detrimental to galling resistance.
Fig. 1 - Galling test arrangement using the Tinius-Olsen machine.
Increasing Si beyond about 5% was considered undesirable, since higher levels of Mn and Ni would be required to maintain a fully austenitic structure, unnecessarily increasing the cost of the alloy. Also, increasing silicon decreases nitrogen solubility; thus high Si would require an additional increase in nickel.

Based on this research, a new stainless steel - Gall-Tough Stainless was defined with superior galling resistance compared to a standard alloy, like Type 304. Nominal composition was 0.10%C, 5.5%Mn, 16%Cr, 5%Ni, 3.5%Si, 0.15%N, Fe-Bal.
Fig. 2 - The efect of silicon and nickel + 0.5 manganese contents on the threshold galling stress values (three rotations) of experimental alloys evaluated. (Numbers by symbolds are threshold galling stress values in ksi.)
Gall-Tough PLUS Stainless Development
Corrosion testing of experimental high-silicon stainless steels determined the corrosion resistance was comparable to Type 304 in most environments. Like Type 304, these alloys were susceptible to chloride pitting corrosion. For manyenvironments, e.g. coastal, brackish water and northern climates where road salts are used, resistance to chloride is required. These environments often use Type 316 which contains molybdenum for improved pitting resistance. However, galling can occurwhen metal parts are in contact under a heavy load. Thus, experimental work was performed to develop an alloy with excellent galling resistance and improved resistance to chloride pitting corrosion.

Table 2 lists the composition of four high-silicon stainless steels. Heat 1 is a laboratory heat of Gall-Tough Stainless, while heats 2 through 4 are experimental modifications. The three modifications have increasing Cr for corrosion resistance, increasing nickel to maintain the austenitic structure, and varying molybdenum (0 to 1 %) contents to evaluate its effect on pitting chloride corrosion. These experimental compositions were induction melted in Carpenter's Research lab and cast as 7.0 cm (23/4 in.) sq. ingots, weighing 7.7 kg (17 Ibs.). The ingots were forged to bar, annealed (1950F/WQ) and galling specimens machined for testing. Also, forged bar was further processed to cold rolled annealed (1950F/WQ) strip, and pitting corrosion specimens machined for testing.
Table 3 lists the threshold galling stress (TGS) and chloride pitting corrosion for these four high-silicon stainless steels. Galling results showed TGS decreased from 15 to 11 ksi with increasing nickel. However, these values demonstrate excellent galling resistance for all these experimental alloys. Pitting tests were performed at RT in 6% FeCI3 for 72 hours per ASTM G48. Tests revealed that increasing Cr reduced the rate of pitting attack while the addition of 1% Mo dramatically improved pitting corrosion resistance.

Based on this research, a second galling resistantstainless steel Gall-Tough PLUS Stainless was defined with excellent galling and chloride pitting resistance compared to Type 316. Nominal composition was 0.10%C, 5.5%Mn, 18%Cr, 8%Ni, 3.5%Si, 1%Mo, 0.15%N, and Fe-Bal.
Production Evaluation - Gall-Tough Stainless and Gall-Tough PLUS Stainless
Table 4 lists the composition of 2.5 cm (1" rd.) production annealed bar of Gall- Tough Stainless, Type 304, Gall- Tough PLUS Stainless, and Type 316. These bars were used to compare galling, wear, strength, toughness and corrosion properties. Tables 4 through 7 list the results of these tests. Galling data shows the poor galling resistance of Type 304 and Type 316 and the excellent resistance of Gall-Tough Stainless and Gall-Tough PLUS Stainless. For best self-mated galling resistance, Gall- Tough Stainless may be the alloy of choice.
Crossed-cylinder wear tests were performed for 40,000 cycles per ASTM G83. Wear data shows the high-silicon alloys have significantly lower wear rates. Gall-Tough Stainless has a wear rate 5x lower than Type 304 while Gall-Tough PLUS Stainless has a wear rate 5x lower than Type 316.

Strength properties showed both Gall- Tough Stainless and Gall- Tough PLUS Stainless have higher ultimate tensile and yield strengths than Type 304 and 316. Yield strengths are 1.5 x greater due to the addition of nitrogen to these alloys. Also, ductility values for all four stainless steels are excellent with elongation and reduction of area)values >50%. Notch impact data shows these stainless alloys are Tough Stainless, Gall- Tough PLUS Stainless and extremely tough with no fractures at 325 joules (240+ ft-Ibs.), the highest strength capable by the test equipment.

Various corrosion tests were performed on Gall- Tough Stainless, Gall- Tough PLUS Stainless and Type 316.
Pitting tests (no crevice) in 6% FeCI3 showed Gall-Tough Stainless and Type 316 pitted severely while Gall- Tough PLUS Stainless had virtually no attack. To increase the potential for attack, a teflon washer was placed on the pitting specimen to produce a crevice site. A test temperature of 0°C was evaluated. Results show only Type 316 showed significant attack. Another pitting test was performed in 6%FeCl3+ 1% HCl at a starting test temperature of 0°C. The temperature was increased until pitting occurred. The higher the temperature at which pitting occurs, the better the pitting resistance. Gall- Tough PLUS stainless had the highest critical pitting temperature. To evaluate the susceptibility to rusting, these alloys were exposed in a 5% NaCI fog at 35C(95F). Less rusting occurred on the Gall- Tough PLUS Stainless samples than Type 316 and Gall-Tough Stainless samples. Corrosion tests in three acid environments determined that Gall- Tough PLUS Stainless has resistance comparable to Type 316, while Gall-Tough Stainless had slightly greater attack.

Customer Application -Bridge Hinge Pins
The Minnesota Department of Transportation, focusing on motorist safety and longer bridge life, specified stainless steel for the link hinge pins (see Figure 3) used in reconditioning the 2431-m (7,975-ft).-long Blatnik Bridge in Duluth. The switch from low alloy steel pins to stainless steel pins, shown in Figure 4, was the first for the department. But it was a logical choice, as these fracture-critical members must possess high- strength, corrosion resistance and galling resistance.
Fig. 3 - Hinge joints, that have been made using two Carpenter Gall-Tough stainless steel pins, allow bridge movement with weather change and traffic load. Fig. 4 - Two of the stainless steel hinge pings that were used on the Blatnik Bridge.

Two pins at the top and bottom of two opposing hanger plates pass through girder webs and act as a hinge that moves with expansion and contraction of the bridge, absorbing the moving load of overhead traffic. The high volume of trucks, buses and other motor vehicles using the bridge makes high strength a paramount requirement for the long term. Generous use of road salt in rugged northern environments, call for the corrosion resistance of a stainless steel. Perhaps more important is movement of the massive steel and concrete structure with changes in temperature. Pins in the hanger assembly are designed to rotate as the bridge sections move. For this reason, the department's structural metals engineer wanted astainless grade that would not gall or wear when rubbing against other metal surfaces under heavy load. Lewis Engineering selected Gall- Tough stainless due to its superior self-mated galling, metal-to-metal wear, coefficient of expansion, and strength characteristics compared to Type 304 stainless.

Both ends of the pins were turned down, and threaded. The mating hanger plates are made from ASTM A-588 steel 4 cm (1 1/2- in.) thick. Threaded, recessed nuts for each end of the hinge pin were made of the same material. Holes for the hinge pins were bored through each hanger plate, finished and coated with a rust inhibitor.

On the bridge, the stainless alloy pins were pushed through the bushing in one hanger plate, through the steel web of the girder suspender, then through the bushing in the hanger plate on the opposite side. Each hanger plate assembly was held in place by recessed steel nuts and 4 cm (1 1/2- in.) cotter pins. High-density polyethylene washers were used as spacers between the hanger plates and pin plates. The hanger plate assemblies shown in Figure 5 were installed on the bridge by ironworkers using hydraulic rams.3 Since the Blatnik Project, Gall- Tough Stainless has been selected by Cardinal Fabricating for several bridge hinge pins applications in the State of Michigan. Coburn Steel Fabrication Inc. had selected Gall-Tough PLUS Stainless for hinge pins that were used in refurnishing bridges in the State of Illinois.

Summary
Stainless steels are being considered as candidates for use in transportation applications where previously coated carbon steels were used. One such application that they were considered a candidate for is bridge components where corrosion problems with hinge pins resulted in expensive retrofitting and safety concerns. Although stainless steels have a higher material cost, life cycle costs are often lower in long-Iife projects, such as bridges. Two new galling resistant alloys, Gall- Tough Stainless and Gall- Tough PLUS Stainless, were designed to minimize galling in applications with unlubricated metal contact under load, a problem which often arises when using Type 304 and Type 316.
Fig. 5 - Hinge plates assembly to girder webs on Blatnik Bridge.
References

1. Magee, J. H, ASM Handbook, Vol. 18, Friction, Lubrication and Wear Technology, p. 715, ASM International (1992).

2. Magee, J. H., Austenitic Stainless Steel with Improved Galling Resistance, High Manganese Austenitic Steel Proceedings, ASM Materials '87, Cincinnati, OH, October 1987.

3. Kuennen, T., Stainless Steel hinge pins anchor bridge retrofit, Road and Bridges, Dec. 1993.
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