Cracking of 7CrMoVTiB10-10 (T24) Steel Weld Joints

Abstract

7CrMoVTiB10-10 (T24) steel was designed for high temperature applications, especially for water wall panels in modern supercritical coal-fired boilers. However, welding of T24 steel caused many problems due to its cracking susceptibility. In the present paper the causes of T24 steel weld joints cracking have been discussed. Microstructural and fractographic analysis of weld joint were conducted and hardness was measured. It was found that the main causes of crack’s formation in weld metal are hot cracking susceptibility of T24, high deformation during welding process and weld metals’ hardenability. The mechanism of cracking was determined. It was concluded that cracks had been initiated in high temperature brittleness range, then propagated through the weld metal in transverse direction as the cold cracks. The reduction of weld metal’s ductility was due to too high cooling rate and excessive hardening of weld metal in relation to base metal.

The Effect of Heat Treatment on the Structure and Hardness of Welded Joints in Steel 7CrMoVTiB10-10

“…Steel 7CrMoVTiB10-10 was launched on the market (and used primarily in the production of waterwalls) as a result of power engineering development, to address issues related to the retrofit of power generation equipment necessitated by an increase in generated power as well as to improve mechanical properties and eliminate the necessity of heat treatment. The steel was obtained through the modification of the chemical composition of steel 10CrMo9-10 (T22), involving the addition of microagents including vanadium, titanium, boron and nitrogen [1,3,4,6,7]. Assumedly, a decreased carbon content in the steel should translate into significantly better weldability and lower hardness, whereas the presence of microagents such as boron, titanium and vanadium should improve creep strength.…”

“…Welds made in the steel contained numerous hot, cold and relaxation cracks. Such a situation could be ascribed to the presence of numerous welding imperfections (of acceptable size), subsequently leading to the propagation of cracks as well as to the negative effect of secondary hardness resulting from the steel composition, the wide range of brittleness temperature, significant deformations of welds as well as the negative effect of hydrogen, present both during the process of welding and during the operation of welded structures [1][2][3][4][5][6][7][8].…”

Structure and Properties of Welded Joints of 7CrMoVTiB10-10 (T24) Steel

“…below 10 mm [7]. Industrial practice showed that welded joints of T24 steel are low ductility [11][12][13][14] and susceptible to cracking [15][16][17], especially cracks in the root pass in butt welded joints of tubes or pipes [18][19][20][21]. Examples of such macro-cracks in the root pass are shown in Fig.…”

Assessing the Susceptibility of Welded Joints to Cold Cracking in the CTS Test

“…The modification of the chemical composition and of the heat treatment of the above-named steel in relation to steel 10CrMo9-10, constituting the design basis for the new grade, led to the obtainment of higher mechanical properties at the cost of weldability. Experience related to the fabrication and operation of tight walls provide information about the high susceptibility of steel 7CrMoVTiB10-10 to cracking, which was described in detail in related reference publications [15][16][17][18][19][20][21][22][23][24][25]. …”