Effect of thermal ageing on fatigue crack growth behaviour of forged alloy 617M at elevated temperatures

ABSTRACT

Fatigue crack growth (FCG) studies have been conducted on compact tension specimens of alloy 617M for as-received and aged for 1000, 5000 and 10,000 h at 710°C. Effects of thermal ageing on FCG behaviour of alloy 617M forged has been characterised at 27°C, 650°C, 710°C and 750°C. Results reveal that threshold stress intensity factor $(\mathrm{\Delta }{K}_{\mathit{th}}$) decreases as temperature increases upto 710°C and then increases at 750°C for all the ageing conditions except for 1000 h aged. The $\mathrm{\Delta }{K}_{\mathit{th}}$ value also decreases with an increase in ageing time till 5000 h, but at 10,000 h a reverse trend is observed. The influence of thermal ageing and temperature reflects major deviations for Paris constant (C) and exponent (m). Additionally, thermal ageing and temperature influences on FCG, the effects of dynamic strain ageing, Cr-rich carbides and γ’-phase precipitations have been discussed and corroborated with fractographic and microstructural changes.

KEYWORDS:

• Ageing
• elevated temperature
• fatigue crack growth
• alloy 617M
• threshold stress intensity factor

Nomenclature

$\mathrm{\Delta }{K}_{\mathit{th}}$	=	Threshold stress intensity factor ($\mathit{MPa}\sqrt{\mathit{m}}$)
C	=	Paris constant $\left(\frac{\mathrm{m}/\mathrm{cycle}}{{\left(\mathrm{MPa}\sqrt{m}\right)}^m}\right)$
m	=	Paris exponent
R	=	Load ratio
W	=	Width of CT specimen (mm)
B	=	Thickness of CT specimen (mm)
$a_0$	=	Initial crack length (mm)
a	=	Instantaneous crack length (mm)
$\mathit{a}/\mathit{W}$	=	Normalized crack size
N	=	Cycle
$\mathit{da}/\mathit{dN}$	=	Crack growth rate (m/cycle)
$\mathrm{\Delta K}$	=	Stress intensity factor range ($\mathit{MPa}\sqrt{\mathit{m}}$)
$\mathrm{\Delta }{K}_0$	=	Initial stress intensity factor range ($\mathit{MPa}\sqrt{\mathit{m}}$)
$K_{\mathit{min}}$	=	Minimum stress intensity factor ($\mathit{MPa}\sqrt{\mathit{m}}$)
$K_{\mathit{max}}$	=	Maximum stress intensity factor ($\mathit{MPa}\sqrt{\mathit{m}}$)
V	=	Voltage drop (Volt)
$V_r$	=	Reference voltage drop (Volt)

Disclosure statement

No potential conflict of interest was reported by the author(s).