Difference between BV FH36 and BV FH40 - Knowledge

BV FH36 and BV FH40 are normalized or thermomechanically processed high-strength shipbuilding steel plates certified by Bureau Veritas (BV). The designation combines key characteristics: "F" indicates they are suitable for Fatigue-critical service, and "H" designates High strength. These grades are specifically engineered for structural members in ships and offshore units where resistance to cyclic loading is paramount, such as in crane-supporting structures, hatch coamings, and critical welded joints in vessel hulls subject to dynamic wave forces.

Key Differences:

The primary distinction lies in their strength level. BV FH36 has a minimum yield strength of 355 MPa (with the "36" historically relating to 36 kgf/mm² tensile strength), while BV FH40 offers a higher yield strength of 390 MPa. This makes FH40 the stronger material, providing a greater safety margin against static yield in high-stress, fatigue-prone areas.

While both grades meet the "F" class requirement for enhanced fatigue performance-mandating controls on steel cleanliness (low sulfur), fine grain size, and improved through-thickness properties-achieving the higher strength of FH40 requires a more refined chemical composition. This typically involves careful balancing of micro-alloying elements (like Niobium and Vanadium) during processing. Consequently, BV FH40 will generally have a slightly higher carbon equivalent (Ceq) compared to FH36. This results in more demanding fabrication requirements for FH40. Welding, in particular, requires stricter procedure controls: the use of qualified low-hydrogen consumables, precise management of preheat and interpass temperatures, and careful control of heat input to preserve the material's fatigue-resistant properties in the weld heat-affected zone. The choice between FH36 and FH40 is a calculated engineering decision, where the incremental strength gain of FH40 must be justified against its higher material cost and the need for more meticulous welding procedures.

Chemical Composition

BV FH36 high Strength Chemical Composition in Tempering and Quenching
Grade	The Element Max (%)
Grade	C	Si	Mn	P	S	Al	N
BV FH36	0.16	0.1-0.50	0.90-1.6	0.025	0.025	0.015
	Nb	V	Ti	Cu	Cr	Ni	Mo
	0.02-0.05	0.05-0.10	0.02	0.35	0.2	0.8	0.08

BV FH40 high Strength Chemical Composition in Tempering and Quenching
Grade	The Element Max (%)
Grade	C	Si	Mn	P	S	Al	N
BV FH40	0.16	0.1-0.50	0.90-1.6	0.025	0.025	0.015
	Nb	V	Ti	Cu	Cr	Ni	Mo
	0.02-0.05	0.05-0.10	0.02	0.35	0.2	0.8	0.08

Mechanical Property

BV FH36 high strength property in tempering and quenching
Grade		Mechanical Property			Charpy V Impact Test
Grade	Thickness	Yield	Tensile	Elongation	Degree	Energy 1	Energy 2
BV FH36	mm	Min Mpa	Mpa	Min %	-60	J	J
	t≤50	355	490-630	21%		24	34
	50＜t≤70	355	490-630	21%		27	41
	70＜t≤100	355	490-630	21%		34	50
Note: Energy 1 is transverse impact test, Energy 2 is longitudinal

BV FH40 high strength property in tempering and quenching
Grade		Mechanical Property			Charpy V Impact Test
Grade	Thickness	Yield	Tensile	Elongation	Degree	Energy 1	Energy 2
BV FH40	mm	Min Mpa	Mpa	Min %	-60	J	J
	t≤50	390	510-660	20%		26	39
	50＜t≤70	390	510-660	20%		31	46
	70＜t≤100	390	510-660	20%		37	55
Note: Energy 1 is transverse impact test, Energy 2 is longitudinal