The hardness of the HAZ is a very good indicator of its susceptibility to cracks and other problems. The hardness at any point in the HAZ can be calculated using the rule of mixtures. Once the volume fractions of ferrite, pearlite, bainite, martensite and austenite are known, then the rule of mixtures can be applied as follows to calculate the hardness [9]:

H - HMXM +HBXB +HAFPXAFP

where

H Total hardness in VPN

HM Vickers hardness of martensite

HB Vickers hardness of bainite

HAFP Vickers hardness of austenite-ferrite-pearlite mixture

XM, XB and XAFP are the volume fraction of martensite, bainite and ferrite-pearlite-austenite mixture respectively. The relationship for hardness calculations of different phases are taken from the work of Maynier et al. [37] and Khoral [4]:

tfM = 127+949C+27Si+ llMn+8Ni+ 16Cr+21(log^) (4-27)

HB =-323 +185C+330Si +153Mn+ 65Ni+ 144Cr+191Mo

+ [89 +53C-55Si-22Mn -lONi -20Cr -33Mo] log Г, HAFP = 42 -223C +53Si +30Mn+12.6Ni +7Cr +19Mo + [10 -19Si +4Ni +8Cr +130V] log Vt

All compositions are in wt%. Vx is the cooling rate at 700°C (°C/h) and can be obtained from cooling time from 800 to 500°C (r8/5):

(4-30)

The value of rS 5 can be measured experimentally or computed from

FEM ox calculated from the Adams [38] relationship for thick plate as follows:

{

1 1

(4-31)

v500-ro 800-Г0

ч t0 uww J0

where

T0 The ambient or preheat temperature (°С)

к The thermal conductivity (J/ms °С)

q/v The heat input (J/m)

It is important to recognize the limits of these equations:

0.1wt%< С < 0.5wt%, Si < lwt%, Mn < 2wt%, Ni < 4wt%, Mo < lwt% Cr < 3wt%, V < 0.2wt%, Cu 0.5wt%, Mn+Ni+Mo < 5wt% and 0.01 wt% < Al < 0.05wt%. They can only be applied to low carbon steels with reservations, and the micro-alloy elements Nb, Ti, Zr and В are not taken into account.

Good agreement between computed and measured hardness were obtained as shown in results by Henwood et al [22]. There is a relative sharp change in the hardness distribution where the peak temperature is approximately 1400°K. Since hardness is very sensitive to the martensite fraction, this would imply that the martensite fraction changes rapidly at this same position in the HAZ contrary to the measurements. A rapid change in martensite implies a considerable change in hardenability. The austenite grain growth causes martensite not vice versa.