Agenda Item 650-560
Title: Revise Low Temperature Preheat Requirements
Date: Oct 4, 2002 (as submitted for SCLB)
April 23, 2003 (as submitted for 2nd SCLB)
Handled By:Doug Miller
Chicago Bridge and Iron Company
1501 North Division Street
Plainfield, IL 60544-8984
Telephone: 815-439-6522
Fax: 815-439-6560
Email:
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Purpose: 1) To create suitable preheat rules to govern welding below 0oF.
2)At the same revise existing rules to better address low-temperature welding of higher strength materials.
Source: CB&I
Impact: Confirms current practices regarding welding when ambient temperatures are below 0F.
Discussion:Existing rules in API 650 and 620 have been interpreted by some readers to prohibit low-temperature welding where it should instead provide guidance on adequate preheat. Prohibiting welding when ambient is below 0F is unnecessarily conservative. On the other hand, some of the pre-heat rules permit welding in low-temperature/high-strength-material combinations that are not good practice. Specifically:
Existing rules in API 650 and 620 indicate that welding shall not be performed when the base metal is below 0oF but does not provide direction on what the acceptable measures are to allow work to proceed when ambient conditions are this cold. Providing an enclosure so that the entire structure is raised to a temperature above 0oF certainly would be acceptable, but less costly provisions can and are used. API should provide rules to govern these practices. Experience indicates that welding mild steel may be done when ambient temperature falls below 0oF given that some preheat is applied.
On the other hand, experience indicates that the real situation needing extra care is in welding high strength material. Therefore the proposed rules increase the preheat threshold temperature for these materials to at least 50F.
This 2nd ballot proposes a somewhat simpler table of rules compared to the 1st ballot as suggested in Bhana Mistry ballot comments. Now the need for preheat is triggered solely by metal temperature and is not tied to ambient air temperature.
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Proposed Change:
5.2.1.2 No welding of any kind shall be performed when the surfacesof the parts to be welded are wet from rain, snow, or ice; when rain or snow is falling on such surfaces; or during periods of high winds unless the welder and the work are properly shielded. Also, no welding of any kind shall be performed when the temperature of the base metal is less than –20°C (0°F). When the temperature of the base metal is–20°C to 0°C (0°F to 32°F) or the thickness of the base metal is in excess of 32 mm (1 1/4 in.), the base metal within 75 mm (3 in.) of the place where welding is to be started shall be heated to a temperature warm to the hand (see 5.2.3.4 for preheat requirements for shell plates over 38 mm [11/2 in.] thick). preheat shall be applied when metal temperature is below the temperature required by Table 5-XX. In that case the base metal shall be heated to at least the temperature indicated in Table 5-XX within 75 mm (3 in.) of the place where welding is to be started and maintained 75 mm (3 in.) ahead of the arc.Table 5-XX – Minimum Preheat Temperatures
Material / Thickness (t)Group per / of Thicker Plate / Minimum Preheat Temperature
Table 2-3 / mm (in.)
Groups / t <= 32
(t <=1.25) / 0 C
(32 F)
I, II, III &IIIA / 32 < t <= 38
(1.25 < t <= 1.50) / 10oC
(50oF)
t > 38
(t > 1.50) / 90oC
(200oF)
Groups / t <= 32
(t <=1.25) / 10oC
(50F)
IV, IVA, V & VI / 32 < t <= 38
(1.25 < t <= 1.50) / 40oC
(100oF)
t > 38
(t > 1.50) / 90oC
(200oF)
5.2.3.4 For circumferential and vertical joints in tank shell courses constructed of material more than 38 mm (1 1/2 in.) thick (based on the thickness of the thicker plate at the joint), multipass weld procedures are required, with no pass over 19 mm (3/4 in.) thick permitted. A minimum preheat of 90°C (200°F) is required for these welds.
5.2.3.5 The requirements of this section shall be followed when welding to Group IV, IVA, V, and VI materials. Permanent and temporary attachments (see 5.2.1.10 for information on shell-to-bottom welds) shall be welded with low-hydrogen electrodes. Both permanent and temporary attachments shall be welded in accordance with a procedure that minimizes the potential for underbead cracking. The need for preheat shall be considered when the procedure is selected either for welding thick plates or for welding during low atmospheric temperature. The welds of permanent attachments (not including shell-to-bottom welds) and areas where temporary attachments are removed, shall be examined visually and by either the magnetic particle method or the liquid penetrant method (see 6.2, 6.4, or 6.5 for the appropriate inspection criteria).
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