Low Alloy Steel Grain Size: will vary and will depend on the type of heat treatment in whichthe steel had been supplied / manufactured to the order specifications. Normally it will be fine grained. You can expect fine grains of ASTM No; 5-7. Etchant for low alloy steel with 0.5% Molybdenum: Normally 2% Nital (2% Nitric Acid and Ethyl Alcohol) with nitric acid concentration of 2% should do for etching. If required you can increase concentration if etchant takes long time. unless you are specific to know about some morphological change determination of Molybdenum bearing phases C.V.Srinivasan Nishi Engineers Pvt Ltd Chennai 20 India June 03 >Iam interested in Metallograhy of Low Alloy Steel (0.5 % >Molybdenum) what should be the ideal grain structure of this >material can anyone tell me what will be the etchant in this >process. > >Share the knowledge

BFW Boiler Feed Water tube sheet should normally be over-laid with Inconel or S.S over lay welding at the tube sheet - at least on the face which is seeing the maximum heat of hdyrogen gas-mixture at 380-400 deg centigrade. I do not think a straight low alloy steel tube sheet including P 5 grade material can be used without over lay welding, as A 182 -P 5 tube sheet also will develop cracks due to delayed hydrogen embrittlement after some years. Even P 5 tube sheet cannot stand the temp of gas mixture coming at the exit of ammonia converter if it is located at the down stream side with high hardness. Since low alloy steel without over-lay welding cannot take the heat, re-check your design before putting to use. Tubes used for BFW boiler feed water at the ammonia converter exit if it is located at the downstream side, then it should be P22 or P3 or better still ASTM A 335-P5 grade material to stand the high temperature hot hydrogen rich gas at 350-370 deg cent. Only "U" tube exchangers are used at the exit of Ammonia Converter down-stream end for BFW Boiler exchanger where shell side steam is generated and tube side hot hydrogen rich gas is used. If the tube is already hard at the "U" bends, it will fail within the first yer of operation - if BFW echanger as you have described - is at the outlet of ammonia converter. You have to first do complete stress relief anneal of the entire tube bundle if you wish to avoid frequent failures in service after the first year of operation at this service temp. If tube to tube sheet welding had been done without taking care of alloy steel welding problem (P22/P3/P5 materials used for such exchanger tubes are air hardening grade materials) by the fabricator of exchanger, you will experience all sorts of porosity, minor cracks (which you cannot notice) L.P. L.F. From this description of BFW exchanger it is unfit for use for such critical operationin BFW Boiler Feed Water service immediately (in my opinion). I would do a thorough examination starting from design verification, material order specification, material certificates received, approved fabrication practices given by manufacturer as per relevant code practices, who did the III party inspection stage-wise, how such high hard "U" tubes could be left out before inserting the tubes in the bundle, cause for such poor quality welding and why III Party inspection did not point out earlier, why aspects such as over-lay welding with either Inconel / S.S and why post weld heat treatment aspects have been omitted. I shudder to think of putting such quality exchanger to use (poor quality executed exchanger) for such critical use such as BFW Boiler water exchanger at the down=stream of ammonia converter. Finallly, It would be better to think of asking specific questions rather than use such forum to get all your problems solved . If you need any consultancy on this, please do not this forum to raise all your doubts. C.V.Srinivasan Nishi Engineers Pvt Ltd Chennai 20 India June 04 e-mail: nishi@hathway.com June 04, 2004 >The part which is under consideration is a forged tubesheet. >We haven't tested it in service as it has been newly >fabricated but we want to eradicate all doubts regarding its >metallurgy as it will be installed in a critical service >i.e. BFW Boiler Feed Water. >What could be tha chances of metallurgical abnormalities in >this part during its manufacturing ? > >We are also experiencing high values of hardness at the tube >U bends of this same exchanger shall there be a limit of >hardness values for U bends in any standards as far as i >know ASTM doesn't cover it. > >Welding problems in tube to tube sheet joints have also been >checked distructively and some was found with weld >abnormalities like porosity, lack of penetration, lack of >fusion etc. Any comment on the cause of these abnormalities >in welding ? > >Adnan Hussain