PSI - Issue 20

Mbelle Samuel Bisong et al. / Procedia Structural Integrity 20 (2019) 37–41 Mbelle Samuel Bisong et al. / Structural Integrity Procedia 00 (2019) 000 – 000

38 2

1. Introduction The formation of cold cracks is often connected with the overheating in the heat affected zone (HAZ) of a part or a sample in which the adverse combination of structural factors, such as intensive growth of grains with high level residual stress and strain, and rather high content of hydrogen observed. Therefore one of the relevant directions of actual researches is the assessment of structural heterogeneity, mechanical properties and damage of HAZ in a weld joint, as pointed by Markashova et al (2019). This is very important especially when welding equipments are operating in extreme climatic conditions such as the North of Russsia, the Arctic and the Subarctic regions. The most widely applied in-process tests of weld joints of low-alloyed high-strength steels for the assessment of cold cracking sensibility are the hard tests named “ Likhaysky ” and “Tekken”, suitable standardized in Industrial Welding Handbook (1979). Revelation of cracks from these tests occurs as a result of action of high shrinkable tension in a root pass of a weld joint by the stress concentration due to lack of fusion. However if in the “ Likhaysky ” test the crack resistance criterion serves the maximum depth of cuts with which the cracks do not appear, then in the “Tekken” test criterion serves a critical cooling speed, as shown by Derlomenko et al (2010). Tough in-process test with a unilateral bevel of one edge causes the high stress concentration in a weld root. In this case the crack usually arises in the large grain area located directly abroad fusion weld and extends both on weld metal and HAZ.

Nomenclature B

bainite, an aggregate of iron carbide and ferrite hydrogen content, cubic cm per 100 grams of metal cold cracking susceptibility indicator forweld vertical section, % cold cracking susceptibility indicator for weld root, % cold cracking susceptibility indicator for weld surface, % diagonal of a print of a diamond pyramid in microns

C

C H C R C S

d F

ferrite, bcc form of pure iron

heat affected zone heat affected zone

HAZ

H C HV

Vickers hardness (diamond pyramid hardness)

HV max maximum value of Vickers hardness I W welding current, A L weld length M

martensite, a hard and very brittle solid solution of carbon in iron

preheating temperature, Celsius degrees

T pr U W V W

welding voltage, V

welding speed, mm/s W 600/500 weld cooling rate from 600 to 500 Celsius degree, °C /s

2. Materials and equipment In order to study the influence of cooling speed in the temperature range between 600 to 500 Celsius on the microstructure of metal and its hardness in a weld joint and its tendency to formation of cold cracks, the hard tests were chosen for steel 14H2GMR at the probes sized of 200 150 16 mm. The weld was done by 4 UONI-13/55 electrodes. Before welding, the electrodes were annealed at a temperature of 420 Celsius within two hours that provided hydrogen level in a metal, approximately equal C = 3 cm 3 /100 g. Hydrogen content was determined by a glycerine method. The welding conditions were chosen as follows: I W = 180 A, U W = 22 V, speed of welding of V W = 2.5 mm/sec. One sample was performed by austenitic weld and OZL-18 electrode. Control of a thermal cycle of HAZ was done by means of the chromel-alumel thermocouple, with a diameter of 0.4 mm. For the micro analysis, the area along the weld joint across the crack was chosen.