What Is Yield Strength?

Quick Answer: Yield strength is the stress at which a material begins to deform permanently. Below the yield strength, the material returns to its original shape when unloaded (elastic behavior). Above it, deformation is permanent (plastic behavior). For most piping materials, yield strength is determined using the 0.2% offset method per ASTM E8.

The 0.2% Offset Method

Most engineering alloys do not exhibit a sharp yield point. Instead, the transition from elastic to plastic behavior is gradual. The 0.2% offset method defines yield strength as the stress at which the material has undergone 0.2% permanent strain:

1. Draw the stress-strain curve from a tensile test
2. From 0.2% strain (0.002) on the x-axis, draw a line parallel to the initial elastic slope
3. The intersection of this line with the stress-strain curve is the 0.2% offset yield strength

Some low-carbon steels exhibit a distinct upper and lower yield point (a visible “drop” in the curve). In this case, the lower yield point is typically reported.

Yield Strength Values for Piping Materials

Material	ASTM Spec	Yield Strength (min) MPa	Yield Strength (min) ksi
A106 Gr. B (CS pipe)	A106	240	35
A105 (CS forging)	A105	250	36
API 5L X65 (line pipe)	API 5L	450	65
A335 P11 (alloy pipe)	A335	205	30
A335 P22 (alloy pipe)	A335	205	30
304L SS (A312 TP304L)	A312	170	25
316L SS (A312 TP316L)	A312	170	25
Duplex 2205 (S31803)	A790	450	65
Super Duplex 2507	A790	550	80
Inconel 625	B444	414	60

Role in Piping Design

Yield strength directly determines the allowable stress for piping design per ASME B31.3:

Design Factor	Formula	Purpose
Allowable stress (hot)	Lower of: UTS/3 or YS/1.5	Sets maximum hoop stress for pressure design
Minimum wall thickness	t = PD / (2SE + PY)	S = allowable stress (derived from YS and UTS)
Hydrostatic test pressure	P_test = 1.5 x P_design	Must not exceed 90% of yield at test temperature
Bolt design	Bolt load = YS x stress area x target %	Typically 50-60% of yield for bolt preload

Common Mistake

Yield strength decreases significantly at elevated temperatures. A106 Gr. B has a yield strength of 240 MPa at room temperature but only 140 MPa at 400°C. Always use the ASME allowable stress at the design temperature, not the room-temperature value from the MTR.

Yield Strength at Elevated Temperature

Material	YS at 20°C (MPa)	YS at 200°C (MPa)	YS at 400°C (MPa)
A106 Gr. B	240	195	140
A335 P22	205	185	160
304L SS	170	110	90
316L SS	170	115	95
Duplex 2205	450	330	275

These values are approximate and sourced from ASME BPVC Section II, Part D. The reduction in yield strength at temperature is the reason why high-temperature piping requires alloy steels with superior creep resistance rather than simply higher room-temperature strength.

Yield Strength vs. Hardness

There is a general correlation between yield strength and hardness, but the relationship is not linear and varies by alloy system. Hardness testing (Brinell, Rockwell) is often used as a quick field verification of material properties, but it cannot replace tensile testing for acceptance per ASTM specifications.

Read the full guide to steel corrosion