ASTM A556 Standard Specification for Seamless Cold-Drawn Carbon Steel Feedwater Heater Tubes

Get Certified ASTM A556 Testing for Reliable Feedwater Heater Tube Performance

A feedwater heater tube is subject to repeated thermal cycles and pressure from the flowing water and steam. A tube that does not meet a specification therefore risks causing a failure and subsequent removal from service of the heater. ASTM A556 provides a standard by which manufacturers and buyers can assess the quality of tube prior to its installation in the feed water circuit of a power plant. Producers use it to control the quality of their cold-drawn and heat-treated production, while buyers utilize it to verify that the received lots meet the mechanical and dimensional requirements for the intended application.

The scope of ASTM A556 includes-

• Material type: Seamless cold-drawn carbon steel tubing, produced in grades A2, B2, and C2.
• Purpose: Establishment of minimum wall-thickness requirements and quality control of seamless cold-drawn carbon steel tubes for use in tubular feed water heaters, including tubes formed into U-tubes.
• Properties evaluated: Chemical composition, tensile strength, elongation, flattening, flaring, and hardness resistance, hydrostatic pressure integrity, and freedom from defects.
• Testing method: A chemist verifies chemical composition against the specified grade, and a technician performs tensile, flattening, flaring, hardness, and hydrostatic testing on representative specimens; each finished tube also passes through nondestructive examination after the final cold-drawn pass and heat treatment to detect defects throughout the entire cross-section.
• Results: Tensile strength and yield strength reported in MPa (psi), thickness and outside diameter in mm (in.), and hydrostatic test results as pass/fail for the test pressure.
• Test limitations: The specification covers tubing sizes from 5/8 to 1-1/4 in. [15.9 to 31.8 mm] outside diameter only, with a minimum wall thickness of 0.045 in. [1.1 mm]; values in inch-pound and SI units are treated as independent standards rather than converted equivalents, and combining values from both systems can produce nonconformance.
• Use conditions: Testing and inspection in accordance with the requirements of ASTM A556 are performed on tubes in the as-finished, heat-treated condition, supplied straight or bent into U-tube form.
• Applications: Procurement and inspection of shell and tube feedwater heater tubing for power plants, boilers, and other high-temperature and high-pressure water circuits.

What are the Uses of ASTM A556 Testing?

ASTM A556 gives tube manufacturers and power plant operators a comparable quality basis that a general tubing specification cannot supply for this specific service. The specification-

• Establishes minimum thickness requirements and dimensional tolerances for feed water heater tubes.
• Verifies the chemical composition and mechanical properties suitable for high-temperature and high-pressure service.
• Confirms hydrostatic integrity of the U-tube when U-tube testing applies.
• Supports quality control during cold-drawing and heat-treatment production.
• Applies nondestructive examination to detect defects across the tube’s full cross-section after finishing.
• Sets dimensional and curvature requirements specific to U-bent tubes.
• Provides a shared procurement basis for buyers and manufacturers of feedwater heater tubing.

Which materials can be tested under ASTM A556?

ASTM A556 applies to seamless cold-drawn carbon steel tubing for use in tubular feedwater heaters, available in A2, B2, and C2 grades. The standard covers straight and U-bent tubes, as long as the final product complies with the established outside diameter and minimum thickness tolerances. Before ordering tubing, an engineer must therefore consider the grade and whether U-bending is required, as well as any optional requirements noted in the specification that might apply to the intended application and therefore must be included in the purchase order.

Why is ASTM A556 Important?

A feedwater heater tube sits in continuous contact with high-temperature water and steam, cycling thermally as the plant operates, so a tube that falls short on wall thickness or mechanical strength risks leaking or failing in service. ASTM A556 therefore establishes minimum wall thickness and mechanical performance requirements for the tubes, as well as nondestructive examination of each tube after the final cold drawing pass and heat treatment, rather than spot checking of samples. For U-tube applications, the standard also controls the geometry of the bend. In U-tube applications, the standard also controls the geometry of the bend through dimensional requirements. Compliance with ASTM A556 therefore reduces the risk of tube failures, which would result in the removal of the feed water heater from service.

ASTM A556 Equipment and Sample Preparation Guide

ASTM A556 requires equipment for chemical analysis, mechanical testing, dimensional inspection, hydrostatic testing of U-tubes, and nondestructive examination of the produced tubes.

ASTM A556 Testing Process and Data Collection

An analyst confirms the grade and size of the produced lot. A chemist or qualified analyst verifies the chemical composition, with product analysis performed on the lot when required by the purchase order. Technicians perform applicable tension, flattening, flaring, hardness, and dimensional examinations in accordance with the specified sampling frequency. For U-tubes, the technician performs the required hydrostatic test and examines the tube for leakage. Each tube is then subjected to nondestructive examination following the final heat treatment after the final cold-drawn pass. An analyst compiles the applicable mechanical, dimensional, hydrostatic, and nondestructive examination results with the tube identification and lot information.

Common Challenges and Troubleshooting

Variation of wall thickness below the minimum specification is most likely the result of the cold-drawing process, and a technician should therefore measure the thickness at multiple points on the tube. Failure of the flattening or flaring test indicates insufficient ductility, which may be caused by improper heat treatment, and an analyst should therefore review the heat treatment history. A hydrostatic test failure indicates the presence of a leakage path that should also be found by the nondestructive examination, and a technician should therefore verify that both procedures detected the issue. For U-tubes, dimensional and curvature requirements help control the geometry of the bend, and a technician should therefore verify the leg spacing and curvature prior to release. If the surface condition of the tubes does not comply with the requirements of workmanship, a technician should review the heat treatment and finishing process before releasing the product.

• The technician records tube identification, grade, outside diameter in mm (in.), and wall thickness in mm (in.).
• The analyst reports tensile strength and yield strength in MPa (psi) and elongation as a percentage in accordance with the tension test.
• The technician records flattening and flaring test results as pass/fail in accordance with the applicable requirements and hardness test results in the applicable scale.
• The technician records applicable hydrostatic test results in accordance with the requirements of the product form and specification and nondestructive examination results as free of, or with indication of defects.
• The report treats inch-pound and SI values as independent standards rather than converted equivalents, consistent with the specification’s own unit requirement, and notes any supplementary requirements the purchase order specified.

Link to ASTM A556