Chuck_Steak

Hey Guys, I wanted to throw my 2C in on this discussion. By the failure circumstances, i'm going to say they failed due to Hydrogen Embrittlement (HE). Hydrogen embrittlement occurs when a susceptible material containing absorbed atomic hydrogen is subjected to a sustained static tensile stress. Steels at hardnesses greater than approximately 30 Rockwell C are susceptible to hydrogen embrittlement, with the susceptibility increasing with increasing hardness. The sustained static tensile stress is likely the result of the residual stresses from the heat treatment procedure. (Most likely it was done properly, but residual stresses are going to be present). The absorbed hydrogen is the result of the vinegar cleaning. HE cracking typically occurs within approximately 48 hours of the introduction of the hydrogen (In this case, 2 days after the start of the vinegar cleaning), so the timing is pretty much right. HE is well known in the plating industry because Grade 5 and Grade 8 fasteners are heat treated to high enough hardnesses to be susceptible. In that case the acid is from the electroplating process, however, Hydrogen can be absorbed from pickling or other acid cleaning processes. To prevent hydrogen embrittlement, a hydrogen relief baking treatment is usually specified, which is typically performed at 400F for 4 hours, within 4 hours of the introduction of the hydrogen. So if you acid cleaned the parts for a couple hours, you would then bake the part out to diffuse the hydrogen from the microstructure. If the part is in the acid for two days, then you don't really have a chance to bake the part out. If you still have the failed part, PM me, I could take a look at it on our SEM and tell you whether it was hydrogen or a quench crack or something different, assuming the fracture features weren't completely destroyed by the cleaning process after the cracks occurred. Basically I'd just open up one of the cracks and see what the fracture morphology looks like. Heck, as long as I was messing around, I'd probably polish and etch a cross section and show you what your microstructure looks like.

With Flag Poles and similar, its not the one large load that you have to worry about, its the accumulation of 1000's of small loads less than the UTS, such as from wind, that will get you. A couple years ago I had to magnetic particle inspect 50+ light poles in a shopping center parking lot. Two poles had fallen over over approximately 2 years. We did a failure analysis and it was high cycle fatigue that propagated from a weld toe. The problem was that the lamps they hung off the poles was very heavy and large. The poles had more than sufficient strength to hold the lamps up, but the accumulation of wind over 3 or 4 years lead to the fatigue cracking. Every pole in the parking lot exhibited fatigue cracks, some propagating through more than 50% of the pole circumference. Probably not a factor in this situation, but something to consider when large projects are going to be suspended for long periods of time.

Stainless Steel essentially breaks into 5 general categories and the following is an excerpt from some of my work training notes: Austenitic: These types have sufficient nickel and/or manganese to remain austenitic at room temperatures. These are the 2XX and 3XX series of stainless steels. They are nonmagnetic and can only be hardened by cold working. In the annealed condition, they have relatively low strength, but exhibit good toughness at all temperatures, including cryogenic. Austenitic stainless steels have generally low resistance to chloride induced stress corrosion cracking. Martensitic: These types are normally alloyed only with chromium, and possibly small amounts of nickel or molybdenum. These are part of the 4XX series of stainless steels. They are magnetic and can be hardened by conventional heat treatments, such as quenching and tempering. Depending upon the hardness (strength) level, these types are susceptible to hydrogen embrittlement. Because these steels can be heat treated, a wide variety of strength and toughness levels can be obtained. Ferritic: These types are also normally alloyed only with chromium, and possibly small amounts of nickel and molybdenum. In comparison to the martensitic grades, however, they have higher chromium contents and lower carbon contents. These are also part of the 4XX series of stainless steels. They are magnetic, but cannot be hardened by heat treatment, although they can be hardened by cold working. These types generally have higher tensile properties in the annealed condition than austenitic stainless steels, however, they do exhibit deceased toughness as the temperature decreases. These types are very immune to chloride induced stress corrosion cracking. Duplex: These types are alloyed with chromium, nickel, and normally other elements to produce a mixed microstructure of austenite and ferrite. Duplex stainless steels are normally identified only by brand names, but may have UNS numbers, especially in ASTM specifications. Duplex stainless steels combine many of the positive attributes of austenitic and ferritic stainless steels, including higher strength and stress corrosion cracking resistance of the ferritic types and improved corrosion resistance of the austenitic types. Precipitation Hardening: These types can be hardened by a low temperature (generally 900 to 1150 °F) aging process. Therefore, they can normally be machined to finish size before aging with minimal distortion or dimensional changes. The mechanical properties can be varied depending upon the aging temperature to properties similar to the martensitic grades, but with corrosion resistance similar to the austenitic grades. Therefore, they are commonly used when high strength and good corrosion resistance are required. These types are alloyed with chromium and nickel, along with elements that can be dissolved during the solution treatment and can be subsequently precipitated during the aging process, including copper, niobium, tantalum, and aluminum. These types are commonly identified by brand names, but are designated as Type 6XX in ASTM specifications. The magnetism of the stainless steels is dependent on the microstructure (as previously stated), and the microstructure is varied by heat treatment and chemistry. I thought the general grade info may be useful for reference.

If it were my anvil, I'd use 4340. You can get very good hardness out of it and it has good hardenability. The nickel alloying addition results in very good toughness as well. I would cast the thing, shape it, grind, clean up the hardy hole, drill the pritchels, and then I would induction harden the face of it from horn tip to horn tip (i'd make mine a double horn german pattern). I'd probably have it induction hardened to about 58 Rockwell C with a depth of around 1/2" or a bit deeper. This should result in a very durable anvil without having to go to a higher alloy tool steel. From the hardnesses that I've seen typically reported for anvil faces, I don't think you would have to go much over a xx40 steel in terms of the carbon content. So at that point your alloying elements just need to be adjusted so you can get the hardened depth and properties you want. I would guess 4140 would work just fine, but if you had the option, the nickel in 4340 would make the face more chip resistant in the long run if the anvil may be subject to some unforeseen abuse.

None of the sickle teeth i've worked with have been carburized or hard faced. Many of the combine or big chopper blades i've seen, however, have been either carburized or have had a thermal spray hard facing. Slightly off topic but.... The hard facing is kind of interesting because it makes the blade self sharpening. The hard facing is generally a tungsten carbide in a cobalt or nickel matrix so it is really really wear resistant. The thermal spray will be applied to the flat cutting edge. The bevel is made in the blade steel and the steel is left without a hard face or case hardening. This results in the blade material wearing away along the bevel, but the hard facing stays intact. So the small layer of hard facing effectively acts as a sharper edge, maintaining the cutting edge. So with regards to how this pertains to forging...If the blade has a slightly rough layer on the flat side adjacent to the cutting edge, its probably a thermal spray and may forge funny. (chip, fall off, etc). Doesn't seem like it would be a big deal tho. Might make an odd welded billet though.

MO, I think the hardness and material is just a compromise to get good toughness and the cheapest material. I don't know the specifics for the material choices, as we are a third party lab. The blades are still relatively tough (we also do some CVN's of the blade materials) but have the hardness and wear resistance for good performance in the field. You could probably make a better blade out of a higher carbon or high nickel alloy steel or a virtually indestructible blade out of a tool steel, but for a replaceable component, the 1038 works satisfactorily and it keeps the price down on a large scale production.

I do chemistries at work on the strip material that is used to produce the the triangle blades for a sickle bar. The blades are 1038 carbon steel and the are heat treated to around 40 HRC.

It sounds like the part probably isn't completely quenching out to martensite. A properly quenched and tempered alloy steel should have a yield to tensile strength ratio of greater than 90%. It looks like your connecting rod is closer to 50%; I'd say your quench severity needs to be increased to assure that you are getting a fully martensitic microstructure. Like Tim mentioned, maybe an agitated quench or else a colder quench bath. I also think a metallographic section would be the best method to confirm whats going on. If the problem is on numerous parts or this is the first time making the parts, it may be the process parameters. If you've been making the part for years and are having isolated problems, then it may be worth going through logs and checking temperature records, etc and also it would probably be worth confirming the chemical composition of the stock. Good Luck

Hello Everybody, Thank you to all for the great community that you have created. I have been lurking for a while around here and finally have a start to post about. I've been intrigued by blacksmithing as long as I can remember, which probably has influenced my career so far. I am a metallurgist and work for a small failure analysis and test lab. Last October, I was fortunate enough to get into the beginning blacksmith class at Old World Wisconsin. It was a great experience and solidified the fact that I want to play around as a blacksmith. My instructor for the class was Darold Rinedollar. He was a great instructor and we had a blast. Hopefully we picked up some good habits along the way. I have a couple pictures of the forges at old world wisconsin attached. We made an S hook, a wall hook, practiced a hey penny scroll, made a door stop and even got a chance to practice forge welding on some scrap. Prior to the class I got a small Vulcan anvil (it was free) and a 4" post vise (also free) so I had some basic equipment. I put together propane forge over the past month or so. Its not 100% complete, I have ITC on order to coat the Kaowoal (don't want silicosis of course), and I need some soft fire brick for the front. This past weekend, however, I finally was ready to test my burner so I fired it up and decided to do some simple forging to give it a try. It took a couple minutes and quickly brought up my stock to orange heat. I will need to tweak it to get hotter, but coating it should help with that. Anyway, I banged out a pair of tongs and bent a quick cutting plate for the anvil (not that the face of the anvil needs to be saved, its gouged to crap, but I want to be in the habit). The tongs are a little rough and I didn't get the hinge geometry quite right, but good practice for next time. I may re-heat the tongs and shape the jaws for round or square stock, but for now I just left them flat. I also broke the edges and polished up the reigns. All in all, it was a very educational and relatively productive test of the forge. Lots of tools to make and fun to be had... Thanks again!