Hardness Testing Gets Up and Goes
Portable hardness testers have finally come into their own within the past few years. As manufacturers have put more emphasis on in-process, or shop-floor testing, the demand for portable hardness testers has grown, along with broader availability of these devices from a growing number of vendors. While portable testers cannot replace benchtop models, they do indeed complement the immobile variety of testers.
All portable testers are not created equal, and there are several factors that buyers should consider when selecting a portable device: the size and type of the material to be tested and the scale needed.
Portable hardness testers have the advantage of being able to test raw material specimens, or materials used in products or parts that are too large or awkward to be moved to a benchtop model, or which can't be easily supported for benchtop testing, says Bill O'Neill, who does hardness marketing and testing applications for Instron Corp. (Canton, MA).
Although portable testers can be used for large test pieces, Ira Friedman, western sales manager at The L.S. Starrett Co. (Athol, MA), explains that using them on thin and soft materials is not an option. "Basically what you're doing is shooting a steel or carbide ball onto the test piece, and the handheld unit is measuring the velocity coming back. If you have a thin piece of metal, that ball will not bounce back because there is no mass behind it. Without mass, you won't get a reading. On softer metal, the ball won't bounce back either, so there is a problem in getting test readings." Because of their makeup, benchtop models more easily allow a support to be put into place, thereby creating the necessary mass to measure workpieces too thin to be tested by portable testers.
Cock and fire
But the good news, according to Friedman is that the time re-quired to actually perform the test with Starrett's portable models is less than that of a benchtop model. "To take the actual physical test on the portable takes about one second. All you do is push a button, it releases a ball and boom, it automatically displays the results," he says. "With the portable, you can cock it and load it in a second and go on to the next one, so you can do instant, automatic testing. As quick as you can load it, that's as quick as you can take a test, whereas with a benchtop, you have to unload it, bring the piece up to the tester, load it again and set the piece correctly."
Friedman adds that another advantage of portable hardness testers is the impact direction, meaning that testing can be done from any angle, whether the tester is held upside down, at a 45-degree angle or a 90-degree angle. For example, a big bar of steel could not be taken to a benchtop, but with a portable, he says, "you can walk right up to the steel and test it in every spot on which you can get the tester. You could go upside down, go underneath it or go behind it, all within seconds," Friedman points out.
Portable hardness testers are not only advantageous for those walking around large factories doing spot checks, but also for use in testing incoming parts. For example, O'Neill says that steel samples can be tested right on the truck and "returned right on the spot if they don't meet correct specifications," saving the labor re-quired to unload the truck only to realize once the parts are un-loaded that they are not up to par.
With a wide range of portable hardness testers available, it is difficult choosing the right model for a company's needs. Some models include software for the most popular hardness scales--Rockwell, Brinell, Vickers and Shore. Other models are almost completely mechanical. Of the models that do include software, some include RS 232 output capabilities for downloading infor-mation to a computer. Still other models have built-in printers. Many portables include different impact probes in different shapes to test the hardness in recesses and grooves.
The latest challenge faced by users and the vendors of both portable and benchtop hardness testers that are designed to perform Rockwell or Brinell tests, is the need to replace the steel testing ball with a carbide ball. The changes in ASTM (American Society for Testing and Materials) E 10-01 and E 18-00, for Brinell and Rockwell hardness test methods, respectively, are to im-prove the performance of the tests and align with the methods set forth in ISO standards, according to Ed Tobolski, general manager of hardness products at Instron, and a member of ASTM subcommittee E28.06, the group responsible for the promotion of knowledge and advancement of methods of mechanical testing.
While the revision for E 10 for Brinell testers became final earlier this year, the E 18 revision for Rockwell standards is currently in the final balloting process and should be available within a year, says Tobolski.
The reason for moving from a steel ball to a carbide ball is that "the steel ball simply isn't hard enough," Tobolski explains. "Steel is a relatively soft material in the grand scheme of things," he observes, and for those who must routinely test extremely hard products, a steel ball can be easily damaged after four to six tests. "So you're constantly changing the ball, and if you don't do it, you could end up with incorrect results," Tobolski states. "That's a very easy thing to do and a common error that people run into. By going to the carbide ball, you virtually eliminate that."
Carbide will last longer than steel, but how much longer is determined by the amount of testing a facility does. "For the people who are testing day in and day out, hundreds of tests a day, then a steel ball would be changed almost every day," explains Tobolski. "A carbide ball, you might want to change once a month--in fairly high use cases. Most people don't use it that much and don't have to change it but once a year."
Beyond the initial cost of investing in the carbide balls, the customer faces other challenges. As one way to head off problems, hardness testing vendors should take steps to educate users about the forthcoming changes, experts say. "If you've got a customer in the field who has a machine with a steel ball in it and old blocks that are calibrated with steel balls, and he buys a new block calibrated by a carbide ball, he's going to see a difference, guaranteed," Tobolski points out. "He may see a point difference in some areas of the curve. He may be confused and say, 'What's going on here? The machine seems to be working OK, but I'm not getting the right answer on these blocks.' One way or the other, depending on which combination he has, if he's not aware that the block is calibrated using a carbide ball, he could get confused."
Tobolski suggests that users prepare for the changes in the Rockwell standard now by purchasing the carbide balls and trying them out to check for differences. "It would be nice to know right now if I'm going to have a problem when the changes to the standard come out. I would get some of the carbide balls and put them in the machine just to see what problems that causes me on the actual part that I'm trying to test. At least get a feel for what's coming down the road and decide at that point that maybe it's not significant," says Tobolski. "I would get a head start on the thing and be ready to deal with it, so I know what impact it will have on the things that I do before it actually becomes a requirement."