General Introduction

The alloy 625 tubes, which are widely used for superheater or heat exchanger tubing, are generally manufactured as per ASTM B444 (ASME SB-444) UNS N06625. It is a typical nickel-chromium-molybdenum-columbium alloy, which is commercially known as a proprietary brand of “Inconel alloy 625“. The cold-worked alloy 625 seamless tubes may be furnished in two grades of different heat-treated conditions: Grade 1 (annealed) – tubes are normally employed in service temperatures up to 1100°F (593°C); Grade 2 (solution annealed) – material is normally employed in service temperatures above 1100°F (593°C) when resistance to creep and rupture is required. Generally, unless otherwise specified, Grade 1 will be supplied.

The cold-drawn alloy 625 tubes can be furnished with ground bright surfaces or pickled matte surfaces depending on the requirements of clients. Especially, small-diameter and light-wall tubes of UNS N06625 are widely used for heat exchanger or instrumentation tubing. The “small-diameter” refers to outside diameters 1-1/4″ (31.8 mm) and under, while “light-wall” refers to a tube with specified wall thickness 3% or less of the specified outside diameter.

Chemical Composition Requirements

Carbon: ≤0.10; Manganese: ≤0.50; Silicon: ≤0.50; Phosphorus: ≤0.015; Sulfur: ≤0.015; Chromium: 20.0~23.0; Columbium + Tantalum: 3.15~4.15; Cobalt: ≤1.0; Molybdenum: 8.0~10.0; Iron: ≤5.0; Al: ≤0.40; Titanium: ≤0.40; Nickel: ≥58.0; all in weight percentage.

Mechanical Properties & Heat Treatment

The mechanical properties of Alloy N06625 Grade 1 (annealed) tubes: tensile strength ≥ 120 ksi [827 MPa]; yield strength ≥ 60 ksi [414 MPa]; Elongation in 2″ ≥ 30%. The mechanical properties of Alloy N06625 Grade 2 (solution annealed) tubes: tensile strength ≥ 100 ksi [690 MPa]; yield strength ≥ 40 ksi [276 MPa]; Elongation in 2″ ≥ 30%. The annealing heat treatment shall be conducted at a temperature no less than 1600°F [871°C]. The solution annealing heat treatment shall be conducted at a temperature no less than 2000°F [1093°C], with subsequent stabilization anneal at 1800°F [982°C] minimum to increase resistance to sensitization.

Tests and Inspection

• Hydrostatic Test: Each alloy 625 tube (seamless & cold-worked) with an outside diameter of 3 mm and larger and with wall thickness of 0.38 mm and larger shall be tested in accordance with ASTM B829. The allowable fiber stresses for UNS N06625 Grade 1 and Grade 2 are 30 ksi [207 MPa] and 25 ksi [172 Mpa], respectively.
• Dimensional inspection and visual examination: outside diameter, wall thickness, inside diameter, length, straightness, plain ends, finish & surface appearance.
• Tension testing (ASTM E8) and hardness test (ASTM E18).
• One or more non-destructive tests (NDT) may be specified: PMI, ultrasonic examination (ASTM E213), eddy current examination (ASTM E426/ E571).

Weight of Alloy 625 Tubes

The weight of an alloy 625 tube can be calculated using the equation: W = ( D – t ) * t * 0.0246615 * F * L. W: weight of the tube, kg; D: outside diameter of the tube, mm; t: wall thickness of the tube, mm; F: correction factor, 1.075; L: length of the tube, meter.

Maximum Allowable Stress, Tensile Strength, Yield Strength @ Different Temperatures

Alloy N06625 tubes in the annealed condition are subject to severe loss of impact strength at room temperatures after exposure in the range of 1000°F to 1400°F. When used for applications designed to ASME Boiler and Pressure Vessel Codes, the alloy 625 tubes, made to ASTM B444 UNS N06625, shall be furnished in solution annealed condition. They can be applied to temperatures up to 1600°F. The values of maximum allowable stress, tensile strength and yield strength at different elevated temperatures are listed in below tables.

*Temp.	*MAS	*Temp.	*MAS
-20~100	26.7	1000	19.6
200	24.6	1050	19.5
300	23.4	1100	19.4
400	22.4	1150	19.3
500	21.7	1200	19.3
600	21.0	1250	15.0
650	20.8	1300	11.6
700	20.5	1350	8.5
750	20.3	1400	6.7
800	20.1	1450	4.9
850	20.0	1500	3.8
900	19.8	1550	2.6
950	19.7	1600	1.9

*Temp.: metal temperature, °F, not exceeding; *MAS: maximum allowable stress for ASTM B444 UNS N06625 Grade 2 tubes, ksi.

*Temp.	*T.S	*Temp.	*T.S
-20~100	100.0	700	97.3
200	100.0	750	97.1
300	100.0	800	96.9
400	100.0	850	96.7
500	99.7	900	96.2
600	98.1	950	95.5
650	97.6	1000	94.5

*Temp.: metal temperature, °F, not exceeding; *T.S: tensile strength for ASTM B444 UNS N06625 Grade 2 tubes, ksi.

*Temp.	*Y.S	*Temp.	*Y.S
-20~100	40.0	600	31.5
150	38.0	650	31.2
200	36.9	700	30.8
250	36.0	750	30.5
300	35.1	800	30.2
350	34.3	850	30.0
400	33.7	900	29.7
450	33.0	950	29.6
500	32.5	1000	29.4
550	32.0	-	-

*Temp.: metal temperature, °F, not exceeding; *Y.S: yield strength for ASTM B444 UNS N06625 Grade 2 tubes, ksi.