Metodologías de Software

Various thermal and mechanical treatments are often performed on welds to reduce the re-sidual stresses and distortion. They include preheat, postweld thermal treatments, peening, and so forth. These treatments also change the metallurgical properties of weldments.

5.3.1 Reasons for Treatment

• To restore the base properties affected by the welding heat.

• To modify weld-deposit properties.

• To relieve stresses and produce desired micro-structure in base material, HAZ and weld metal.

• The extent of harm the weld has caused determines the subsequent treatment.

• Improve weldability (for example preheat improves weldability).

• To reduce “metallurgical notch” effect resulting from abrupt changes in hardness etc.

• To improve resistance to crack propagation.

5.3.2 Code Requirements

Some welded constructions are required to be in accordance with the recommendations of a code such as the ASME Boilers and Pressure Vessels Code, thermal treatments are specified

for certain types of weldments. These recommendations are based upon the existing evidence necessitating the thermal treatment. These are codes for minimum requirements. The fabrica-tor should employ other treatments also based upon his experience in addition to the code requirements. Some important codes are given below for example :

1. ASME Boiler and Pressure Vessels Code, Section I, III, VIII Divs. 1 and 2 (latest edi-tion). New Yorlk: American Society of mecanical Engineers.

2. Code for Pressure Piping, Ansi B 31.1 to B 31.8 (latest edition) New York: American National Standards Institute.

3. Fabrication Welding and Inspection, and Casting Inspection and Repair for Machinery, Piping and Pressure Vessels in Ships of the United States Navy, MIL–STD–278 (Ships) (latest edition) Washington D.C. : Navy Department.

4. General Specification for ships of the United States Navy, spec. 59-1 (latest edition) Washington D.C. : Navy Department.

5. Rules for Building and Classing Steel Vessels (latest edition) New York : American Bureau of Shipping.

6. Structure Welding Code AWS D 1.1 (latest edition as revised). Miami : American Weld-ing Society.

7. United States Coast Guard Marine Engineering Regulations and Materials, spec. CG -115 (latest edition). Washington D.C. : United States Coast Guard.

As these documents are constantly revised, the latest available versions should be ob-tained and followed.

5.3.3 Common Thermal Treatments

Preheat. Preheat temperatures may be as low as 26°C for out door welding in winter to 650°C when welding ductile cast iron and 315°C when welding highly hardenable steels. In many situations the temperature of preheat must be carefully controlled. The best way is to heat the part in a furnace and held at the desired temperature.

• Preheating is very effective means of reducing weld metal and base metal cracking.

It retards the cooling rates and reduces the magnitude of shrinkage stresses.

• Also the thermal conductivity reduces as temperature increases (for iron thermal conductivity at 595c is 50% of its value at room temperature). This also reduces the cooling rate resulting in favourable metallurgical structure, HAZ also remains at the transformation temperature for a longer period of time permitting the formation of ferrite and pearlite or bainite instead of martensite.

• When an area being welded is under severe restraint, localized preheat may increase the amount of shrinking and cause cracking.

Thus preheat must be used with caution, since detrimental effects may result under certain conditions.

Electrical strip heaters are commonly used on site for preheating.

These must be properly insulated to avoid danger of shock to welders.

Induction heating, using 60 Hz (or 50 Hz) transformers of suitable capacities built for this purpose, is a common method of preheating pipe joints for welding.

5.3.4 Postweld Thermal Treatment

• Stress relief heat-treatment is defined as the uniform heating of a structure to a suitable temperature, holding at this temperature for a predetermined period of time, followed by uniform cooling (uneven cooling may result in additional stresses).

• Stress relief heat treatment is usually performed below the critical range so as not to affect the metallurgical structure of the work.

• The percentage relief of internal stress depends upon the type of steel (its yield strength). The effects of varying time and temperature are shown in Fig. 5.11.

1 Time at stress relieving temp. = 1h 2 = 4h

315 370 430 480 540 595 650 705

Stress relieving temperature, °C

1

2

3

1 70000 psi yield strength steel 2 50000

Stress relieving temperature, °C (time at temp., 4h)

Fig. 5.11. Effect of temperature and time or stress-relief

• The temperature reached is more effective than the time at that temperature in stress relieving. Temperatures closer to recrystallisation temperature are more effective.

• Microstructure, tensile and impact strength values are affected by stress relief treat-ment. Temperature for stress relief should be so chosen as to develop or retain the desirable properties while at the same time provide the maximum stress relief (Table 5.2).

• Controlled low temperature stress relief treatment could be done when the struc-tures are big enough to be stress relieved in a furnace. The material on either side of

the weld bead is heated to 175°-205°C while the weld itself is relatively cool. This causes thermal expansion in the base metal and a reciprocal tensile stress in the weld beyond the yield. When the metal cools and contracts, the stress falls below the yield.

When the process is used properly a partial reduction in the longitudinal stresses of butt welds is achieved.

Table 5.2. Typical thermal treatments for weldments

Material Soaking temperature

°C °F

Carbon steel 595–680 1100–1250

Carbon–½% Mo steel 595–720 1100–1325

½% Cr–½% Mo steel 595–720 1100–1325

1% Cr–½% Mo steel 620–730 1150–1350

1¼% Cr–½% Mo steel 705–760 1300–1400

2% Cr–½% Mo steel 705–760 1300–1400

2¼% Cr–1% Mo steel 705–770 1300–1425

5% Cr–½% Mo

(Type 502) steel 705–770 1300–1425

7% Cr–½% Mo steel 705–760 1300–1400

9%Cr–1% Mo steel 705–760 1300–1400

12% Cr (Type 410) steel 760–815 1400–1500

16% Cr (Type 430) steel 760–815 1400–1500

1¼% Mn–½% No 605–680 1125–1200

Low-alloy Cr-Ni-Mo steels 595–680 1100–1250

2 to 5% Ni steels 595–650 1100–1200.

9% Ni steels 550–585 1025–1085

Quench & tempered steels 540–550 1000–1025

5.3.5 Peening

Peening has been used by the welding industry for over 35 years, but the code requirements and regulations governing this procedure have been based on opinion rather than on scientific data because there has been no practical method for measuring the effect of peening.

Various specifications and codes require that the first and last layers of a weld should not be peened.

The results of laboratory tests conducted by American Bureau of Shipping and explo-sion tests by the Naval Research Laboratory confirm the requirement prohibiting the peening of the first and the last layers.

In conducting peening, the following special precautions may be necessary:

(1) Work hardening should be considered when certain AISI 300 series steels are involved.

(2) Hot shortness may preclude hot peening of certain bronze alloys.

(3) AISI 400 series steels have relatively poor notch ductility in the as-welded condition.

Utmost care should be exercised if peening is attempted.

(4) The relative elongation values for ductility of welds and metals should be considered before employing the peening process.

Peening equipment should be selected with care The hammer, pneumatic tools, and so forth should be sufficiently heavy for striking force to be effective without producing excessive work hardening, but not so heavy as to involve bending moments or produce cracks in the weld.