EFFECTS OF HIGH TEMPERATURES AND FIT-UP OF RIVETED JOINTS ON STEEL RIVETS
|Publication Date:||2 October 2017|
The objective of this study was to evaluate the effects on rivet properties based on maximum hot rivet driving temperatures, plate fit-up, rivet-hole clearances and joint tightness. The most urgent need was to establish whether heating rivets to 2250 degrees F (1230 degrees C) for driving had a detrimental effect on final rivet mechanical properties compared with rivets heated at 1950 degrees F (1065 degrees C). At present, the 2017 Edition of the BPVC, Section I (1) has limited the maximum temperature for hot riveting to 2250 degrees F (1230 degrees C). However, this maximum driving temperature has been challenged by others in the past who have maintained the maximum temperature should be 1950 degrees F (1065 degrees C).
This study focused on the two temperatures previously mentioned, but added additional variables to the test plates for consideration outside recommended parameters for standard hot riveting practices. These variables included oversizing rivet holes, poor plate fit-up using spring back, and soaking rivets at the higher temperatures 1950 degrees C and 2250 degrees F (1065 degrees C and 1230 degrees C) for extended periods (10 minutes and 20 minutes) before driving into the test plates.
To accomplish the objective, it was decided to simulate riveting of locomotive boilers by using carbon steel test plates 4-inches (102 mm) by 20-inches (508 mm) by ¾-inch (19 mm) in thickness with pre-drilled holes to accommodate carbon steel rivets. The rivets for this entire project were supplied from ASTM 675 carbon steel bar, 7/8-inch in diameter. A pneumatic hammer was used to drive the 7/8-inch (22 mm) diameter rivets into button head style heads after heating to the designated metal temperature for hot forming. Heating of rivets was performed using electric resistance heating and a gas fired furnace. Three test conditions were performed;
The first test condition used carbon steel test plates which were prepared with rivet holes 1/8-inch (3.2 mm) oversized, to see if the additional upsetting of the rivet to fill the larger holes was detrimental to the finished steel rivet mechanical properties.
The second test condition used carbon steel test plates with holes 1/16-inch (1.6 mm) oversized (normal) to simulate the situation where plates are not tightly fit together, thus allowing the plates to spring-back during hot riveting. This test was in response to a specific report in an early study that indicated this cyclic compression/stretch during driving created a sub-par finished rivet.
The third test condition dealt with carbon steel test plates with 1/16-inch (1.6 mm) oversized holes that were tightly held together. In addition, during this third series of tests, the rivets were held for 10 minutes and 20 minutes at metal temperature before driving. A complete spreadsheet summarizing the metallurgical lab tests is shown in Table 1-1 in Appendix A.
A fourth test had been in the original test program where hand riveting is accomplished using hand hammers with no pneumatic or hydraulic assistance. While this may be done on a very limited basis for historical purposes, the scope of its use in Test 4 did not seem relevant enough to justify the expenditure given limited budget constraints.
Upon completion of riveting by Strasburg Railroad, the hot riveted carbon steel test plates were sent to a metallurgical lab under the direction of DTS Metallurgical. Each rivet was carefully extracted from the carbon steel test plates with the material subjected to the following metallurgical lab tests; macrophotographs of extracted rivets, tensile testing using sub-size tensile specimens, hardness testing, and metallographic examination.