Announcement

Collapse
No announcement yet.

How Hard does a valve seat have to be?

Collapse
X
 
  • Filter
  • Time
  • Show
Clear All
new posts

  • How Hard does a valve seat have to be?

    Alright y'all,

    I was trying to persuasively discuss CNG conversions with a friend who works as an induction heating engineer, and I mentioned the need to harden valve seats in order for the conversion to not greatly shorten the engine life.

    We both ended up wondering: is there a Rockwell number for the proper level of hardness in this situation, and in which scale--B or C? Finally, we began to discuss the value of surface hardening (carburetion) versus through hardening, and settled on surface hardening as sufficient.

    Can any of y'all with experience either verify or dismiss our concensus? Also, if anyone has a RH number, I would certainly appreciate your sharing it with us.

    Thanks,


    Andy-Paul

  • #2
    Re: How Hard does a valve seat have to be?

    to answer my own question, Rockwell C 35-40 is a good range.

    Comment


    • #3
      Re: How Hard does a valve seat have to be?

      many manufacturers will harden the seats to around C35-38. When using cng the temp on the seat can reach 1600 degrees. Seat inserts can use stainless, stellite, tribaloy cobalt, copper alloy, or nickle alloy.
      Part of the problem in seat recession for cng is the temp that it works under. Ford has had valve seat problems for many years and has not seemed to be able to fix it. I have repaired Ford engines for seats coming loose and breaking up for a long time & just today am replacing an engine in a 2000 that dropped the seat, broke it into pieces, cracked the piston which cracked the block.
      With cng the hardness is a factor, but dont forget temp rating and heat dissapation. I prefer to use nickle alloy or even a little softer copper alloy to stainless or stellite in cng.
      What do you recommend? I am always looking for better information than I have.
      Stan M.

      Comment


      • #4
        Re: How Hard does a valve seat have to be?

        Stellite 31 is still my favorite, so far, and a leftover from my days of doctoring kilns to burn way outside their ratings. A lot of people would consider it overkill (not to mention expensive!!), but I like a good margin of safety. It has about 10% Ni in the mix.

        "Stellite 31 (Stellite X-40) is a cobalt-base, high-temperature superalloy having high tensile and creep properties in precision casting. It is superior in stress-rupture properties to most alloys commercially available, especially at 1700 and 1800°F. Stellite 31 is resistant to oxidizing and reducing atmospheres up to 1150°C (2100°F). Excellent resistance to thermal and mechanical shock. Mainly used in gas turbine engines in areas subject to hot gas erosion. " (http://www.stellite.com/Technical/Al...2/Default.aspx)

        andy-paul
        Last edited by Andy-Paul; 09-22-2008, 09:39 PM. Reason: forgot something

        Comment


        • #5
          Re: How Hard does a valve seat have to be?

          Isnt stellite is the material used by manufactureres that comes loose in aluminum heads when the engine overheats causing catastrophic failure?
          I use stellite in cast heads when repairing seat recession, but I am not sold on stellite for aluminum heads, or for cng, the expansion rate is different causing the seat to loosen in the head creating expensive problems for consumers. I dont mind as I replace an engine a month from the seats dropping and breaking up, but it is a big hit in the pocket book for consumers.
          Stan

          Comment


          • #6
            Re: How Hard does a valve seat have to be?

            Stellite is a brand name for a whole range of cobalt and chromium alloyedwith other metals (30+, IIRC).

            I don't know which alloy has been giving you fits, but I can tell you that there a lot of mechanical engineers out there who have screwed up in the design phase by not checking in with a professional metallurgist before choosing a given alloy (one of my metals profs used to really hammer mechanical engg students who had the temerity to take his class about how rich he had grown on ME screw-ups).

            Let me ask the sales department over at Stellite about that issue, and see what they have to say. There may be an alloy specifically developed that someone either missed or found too expensive for their liking.


            Andy-Paul
            Last edited by Andy-Paul; 09-24-2008, 10:41 PM.

            Comment


            • #7
              Re: How Hard does a valve seat have to be?

              First bit of info:

              Stellite is not a good idea in an aluminum head for CNG temperatures in the case of system overheating, given what I have seen in comparing thermal expansion data (which I will post as a comparative graph as soon as I get them drawn up). As I find more data to pitch in, I will pass it along.

              any-paul

              Comment


              • #8
                Re: How Hard does a valve seat have to be?

                Thank you from the trenches! This cng business has us working on systems without wiring diagrams, diagnosing problems with no repair manual & figuring out a computer that we have little (or no) information on. The engine part is something I know a little about, I can machine a head and install inserts, buty I have no idea of the metal or expansion eexcept what I read when ordering. Any information will be appreciated and used. This forum is actually pretty useless for a technician, I have mentioned berfore -- there are over 20,000 responses with opinions about installing whatever we want with no oversight -- but if you ask a technical question the people who authored those 20,000 responses have no input. It makes me wonder what standard of quality they use for installation, like galvanized pipe fittings instead of stainless because it is cheaper --- yes I have seen several of those (and worse) I refuse to work on them.
                Stan

                Comment


                • #9
                  Re: How Hard does a valve seat have to be?

                  I have been told by one of my thermal dynamics professors that Inconel is one of the best seat materials that he has worked with. He was one of the founders at Impco and is recognized as such on the Impco web site. I found his advice to be worth while and have had very good luck with Inconel Valve seats, in natural gas and propane engines. I have only used Inconel with cast iron cylinder heads, and would also like to see how they perform with aluminum cylinder heads, but have not yet had the opportunity. I too am worried about the different thermal expansion values, and am unwilling to experiment on my dime. I would seriously consider adding a alcohol/water injection system to any application that may see sustained heavy load conditions, such as towing, or racing. I would never want to see 1600 degree temperatures in the combustion chamber, as the NOx emissions would be through the roof. Also remember what the melting temperture of aluminum is! If you are on the "ragged edge", as Don would say, then you better hope your fuel quality is exceptional. Best of luck and let us know what you find.

                  Comment


                  • #10
                    Re: How Hard does a valve seat have to be?

                    Don Bass is a true treasure, and take his advice to heart. He is one of those true "been there, done that" guys. I consider myself fortunate to have spent some time with him. We have shared quite a few war stories between the two of us.

                    When installing an alloy valve seat in an aluminum head, be very careful when cutting the pocket. Some seats have little ridges to help secure them, while other seats have a reverse chamfer on the outside to lock it in place. Some require a nitrogen bath to shrink, while others press fit in at room temp. And some use a liquid sealer, others install dry. Follow the manufacturers requirements, not a well meaning machinist that says "I've been installing valve seats for 20 years, I know what I'm doing!" All it takes is one ruined seat and the head is shot.

                    Franz
                    Last edited by Franz; 09-13-2009, 03:45 PM. Reason: added material

                    Comment


                    • #11
                      Thing of the past?

                      Kevin and I feel the need for hardened seat valves ought to now be a thing of the past. The seat recession issues everyone faced on CNG engines a number of years ago were primarily due to the fog / carburated systems which were placed on conversions back then. They caused uneven fuel distribution among the cylinders which resulted in some cylinders getting extremely hot while others were cooler.

                      However, if you replace the OEM injectors with CNG ones in the same holes, and also use the OEM controller (modifying the stoic trim calibrations for CNG of course) you end up with temperatures close to or even cooler than the OEM achieved on gasoline.

                      As you can see from the attached thermocouple test we performed recently we were actually able to show cooler temps on CNG than gasoline in the lab, both engine-out and catalyst.

                      It's all about carefully calibrating for the subject engine platform. This is yet another reason why I am so sorry to see folks throwing on illegal "universal" conversion kits, especially if the vehicle is destined for high-load conditions and the natural gas injection is performed via a tapped intake manifold (as many of these bi-fuel imported kits do), not directly into the cylinder head. Seats recede and then everyone believes CNG universally screws up engines.
                      Attached Files
                      Last edited by John Mitton; 09-15-2009, 11:22 PM.

                      Comment


                      • #12
                        Re: Thing of the past?

                        Originally posted by John Mitton View Post
                        As you can see from the attached thermocouple test we performed recently we were actually able to show cooler temps on CNG than gasoline in the lab, both engine-out and catalyst.
                        Now that is cool stuff!

                        Please post some tech pictures of the TC in action. What software was used to graph the temps?


                        ?Innovation is driven by having access to things.? -- Gleb Budman, CEO of backblaze.com

                        Comment


                        • #13
                          Re: How Hard does a valve seat have to be?

                          I have worked on this issue for 9 years and have been testing this in a standard GM ECO-TECH 4 cylinder engine since 2007. Heat is the issue, regardless of valve seat hardness, the cylinder head material around the valve seat is the issue.

                          Comment

                          Working...
                          X