Haz

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I have 5 inch pipe sch 120 CS welded with TIG and SMAW . After welding carried out hardness test, found out that the hardness value on weldment higher compare to HAZ and parent metal. Can anybody refresh me on this.
Reply:Need more info to be certain...type of filler metal and general locations of hardness tests for starters..But, my first guess would be that the filler metal used could be a higher strength material than the base metal.  If so, it's quite possible that the weldment would show a higher hardness than the HAZ or base metal.Higher strength filler (For example:  ER80S-D2 for TIG, or 7018 through 12018 for SMAW) have yield strengths that are as much as 1.5x-3x greater than the base metal you described.
Reply:As mentioned above, the filler metal itself could make the weld harder than the HAZ, but there is also a phenomena known as "HAZ softening" that is seen in some pipeline steels.Generally a plain carbon steel will tend to have increased hardness in the HAZ since a portion of this region transforms from ferrite and cementite to austenite on heating and then forms some harder structure of martensite or bainite upon rapid cooling.  The hardness profile below shows increased hardness in the HAZ.Plain carbon steels rely mainly on substitutional and interstitial alloying for moderate strengthening.  This is where alloying elements stress and strengthen the lattice structure of iron by either taking the place of iron atoms, or by squeezing in between the iron atoms.  Areas of the HAZ that are heated but not transformed to austenite tend to maintain the strengthening (hardening) effect of this alloying.What you may have however is a more sophisticated steel that achieves greater strength through thermomechanical processing and alloying to produce very fine grain size and/or small precipitates among the grains.  Pipeline steels such as X-80, X-100, and X-120 use this type of strengthening.  These steels are suseptable to HAZ softening since regions of the HAZ are heated enough to cause grain growth and precipitate growth.  Here is a very technical paper from the Edision Welding Institute that looks at HAZ softening in these "new" higher strength pipeline steels.http://www.mms.gov/tarprojects/434/434AC.pdfAnd a hardness survey showing HAZ softening in a pipeline steel. Attached FilesHAZ.doc (55.5 KB, 27 views)HAZ softening.doc (99.5 KB, 24 views)Last edited by pulser; 03-05-2008 at 03:02 PM.
Reply:A_Dab, not jumping on your case but 7018 makes a soft ductile deposit. It is used in machine shops for build up rod because it machines so well. 8018 on up, you are right.
Reply:No worries oldtimer.  I appreciate the extra info. Originally Posted by OldtimerA_Dab, not jumping on your case but 7018 makes a soft ductile deposit. It is used in machine shops for build up rod because it machines so well. 8018 on up, you are right.
Reply:Thanks guys, got some ideas on what to look into......