ASarkar HIC is a fine interstitial nature of crack which is difficult to detect except by Ultrasonic Attenuation measurement. UT attenuation method will show loss of back wall echo proportional to the HIC propagation in the lattice (most likely to be inter-granular cracks in nature). Loss in attenuation will be directly proportional and will give the extent of propagation due to HIC. For this you need to have a calibration standard of the same material and same original treatment given (forged, piping or heat treated) that is being tested to detect and compare from original back wall echo with reference to the loss in UT attenuation signal you get due to HIC Hope this helps C.V.Srinivasan Nishi Engineers Pvt Ltd India Jan 6, 2009 E-mail: nishi@vsnl.com >Hi all > >Can u please provide some info on the UT scanning technique >of loss of back wall echo , to detect fine cracks arising >from hydrogen induced cracking (HIC) ? > >Regards

A Sarkar Your question relates to detecting HIC using UT technique. HIC manifests in different forms to attack a material or combines with alloying elements to form different mechanisms of cracking trends. This is a known fact to users who are experienced in NDT, corrosion, metallurgy and plant processes and the mechanisms in which this would occur in steel or in low alloy steels. To answer your question from a reasonable assurance of detection to clearly say, UT attenuation is still possibly the best from detection of HIC - whether low temp or high temp related forms of cracking. Step wise crack, given by Debasis in his reply, is not a fully accepted theory for HIC, even today. It is one of the possible mechanisms for HIC (with nascent hydrogen and other alloying element combining to develop cracking in creating a wedge effect to propagate in highly stressed areas in steel with or without alloying element combinations). UT shear wave technique may help in giving info on possible orientation,length and depth of crack but the NDT engineer would surely be not able to pin pointedly say (by himself or with his limited knowledge on metallurgy, corrosion and material behavior in different process environments) that cracking was due to HIC or H2S induced or Chloride induced or presence of inclusions or other combinations of alloying elements that had contributed to the cracking trend. For this, the person scanning by UT technique need the skill of detecting the crack and then establish with a sound knowledge in metallurgy and corrosion mechanisms in steel to clearly say it was HIC induced at low temp or high temp related. Usually, such a combination with diagnostic and analytical person may not be available in most industries. UT Attenuation technique will detect loss in back wall echo (with comparative evaluation done on a pre-calibrated standard and this would definitely inform the user about the loss and depth of HIC effect in steel or low alloy steel. Hope this helps C.V.Srinivasan Nishi Engineers Pvt Ltd India Jan 23,2009 E-mail: nishi@vsnl.com >Dear Ananda, > >With compliments to Mr. Srinivashan pl. note the following. > >You have to sure whether you are searching for HIC or HTHA. > >HIC which is a typical low temperature degradation >mechanism called wet H2S cracking occurs under acidic >condition which is often a manifestation of hydrogen >blistering. HIC is principally stress independent and >basically depnds on material cleanliness w.r.t laminations , >inclusiond and sulphur% in steel. The cracks are mostly in >the form of stepwise cracking inside the steel. > >HTHA is typically a high tempetaure degradation mechanism >which is temperature and hydrogen partial pressure >dependent. > >To determine HTHA you can refer API 941 Appendix E and can >use a combination of Frequency dependent hydrogen >backscatter technique with velocity attenuation. > >To determine HIC you can use shearwave or normalwave high >frequency UT to determine stewise cracking. >regards >>Hi all >> >>Can u please provide some info on the UT scanning technique >>of loss of back wall echo , to detect fine cracks arising >>from hydrogen induced cracking (HIC) ? >> >>Regards