Ⅰ. Structural characteristics and advantages of Cast Steel Gate Valve

• (1) The cast steel gate valve has a reasonable design, reliable sealing, smooth channel, and small flow resistance coefficient.
• (2) The gate plate and valve seat sealing surface of cast steel gate valve are made of iron-based alloy surfacing welding or Stellite cobalt-based hard alloy surfacing welding, which has good wear resistance, high-temperature resistance, corrosion resistance, scratch resistance, and long service life.
• (3) It adopts a wedge-type elastic gate or rigid wedge-type single gate structure, the rolling bearings are installed in medium and large diameters. It has a flexible switch that is easy to open and close.
• (4) The valve stem of the cast steel gate valve has good corrosion resistance, abrasion resistance, and wear resistance after tempering and surface nitriding treatment.
• (5) It can adopt various piping flange standards and flange sealing surface types to meet various engineering needs and user requirements.
• (6) The valve body of the cast steel gate valve is complete, and the packing and gasket can be reasonably selected according to the actual working conditions or user requirements and can be applied to various pressure, temperature, and medium working conditions.

Ⅱ. Do internal cracks of the cast steel gate valve need to be repaired by welding?

First of all, we need to test the pressure in the water to detect test whether it is qualified or not. Sometimes, we will find some small cracks in the valve body that will affect the use effect of the cast steel gate valve. Therefore, it is necessary to fix these small cracks by welding.

When there is water inside the cast steel gate valve, it cannot be welded if the water inside is not cleaned out first. Also, this welding method is difficult because you have to completely drain the water before welding. It is best to use vertical seam welding due to its thicker welding part. In addition, the amount of cast steel gate valves on the market is very small. For a steel component, there is no cracking after welding.

It is also worth noting that normal welding rods can be used when welding between cast steel and normal carbon steel (such as low-carbon steel). Stainless steel must be matched with welding rods.

ZECO has developed its laboratory for R&D for international standard valve design innovation and validation, including PT, MTUT, and FE testing. ZECO is a modest and united team with more diligence and enterprising to show integrity to customers. If you have any need, welcome to consult us.

How to Weld Cast Iron vs. Cast Steel

Stick welding is a great process to weld cast steel because you have high control over the welding heat. It’s great for quick repairs and fieldwork. Furthermore, it is the best process to weld metals with dirty surfaces.

How to Weld Cast Iron?

If you’re going to tackle welding cast iron, take the following four steps before welding:

1. Identify the alloy
   • The three primary grades of cast iron include gray iron, ductile iron, and malleable iron. Note that there is also white cast iron, but most agree it’s impossible to weld it successfully.
   • Gray iron is the most common form of cast iron, presenting a challenge for welders when the graphite flakes enter the weld pool and cause weld metal embrittlement.
   • Ductile and malleable iron are less brittle because of their microstructural differences. Both have spheroidal carbon microstructures from their manufacturing processes.
2. Clean the casting thoroughly
   • Castings need to be properly prepared before welding. Remove any paint, grease, oil, and other foreign materials from the weld area. Then apply heat carefully and slowly to the weld zone for a short time to remove trapped gas in the base metal.
     An oft-used technique to test the cleanliness of the surface is to deposit a weld pass on it. If impurities remain on the iron, the weld will be porous. You can repeat the process until porosity is no longer present.
3. Preheat the casting
   • The main reason for controlling the heat is thermal expansion. When metal is heated, it expands. If the entire object warms and expands at the same rate, there is little stress. However, it creates stress and cracking when the heat is localized in a relatively small heat-affected zone (HZ). Preheating minimizes the thermal gradient between the casting body and the HZ, lowering the tensile stress caused by welding.
4. Select a suitable welding technique
   • Choose the welding technique that is most suitable for the iron alloy. The three most common processes are stick, oxy-acetylene, and braze welding, with the stick being the most popular.
   • With stick welding, three primary filler electrodes work for cast iron: copper alloy, cast iron covered, and nickel alloy. The nickel alloy electrodes are quite popular for cast iron because the nickel-iron weld is stronger and has a lower coefficient of thermal expansion, reducing stresses and increasing resistance to cracking.
   • Direct the electric arc at the weld pool instead of at the base metal. Doing this minimizes dilution. Use the lowest current setting approved by the manufacturer to help reduce heat stress. Preheat the pieces to at least 250°F before welding with copper or cast iron or electrodes. No preheating is needed with nickel electrodes.

How to Weld Cast Steel?

Learning how to weld cast steel is less of a challenge compared to cast iron. That’s because cast steel is carbon or alloy steel that has been melted and molded into a specific shape. However, even though cast steel has similar properties to rolled steel, some differences make it harder to weld.
Although you can weld cast steel using the MIG or TIG processes, many welding experts recommend stick welding with E7018 rods for low-carbon alloys and stainless-steel rods for hard-to-weld castings.

One of the issues with welding cast steel is avoiding distortion. Here are some suggestions to help with that:

• Preheat the casting: Castings can have irregular shapes, with thinner areas heating more quickly. Concentrate the heat on thicker regions.
• Tack weld frequently.
• Set to the lowest amperage that will produce a strong enough weld.
• Use the fastest traveling speed possible.
• Use fewer passes.
• Slow down the cooldown by wrapping it in welding blankets or burying it in the sand.
• A few more tips on how to weld cast steel successfully:
• Clean the edges to be welded down to the bare metal.
• Preheat cast steel if it has more than 0.40% carbon or it is low-alloy steel. Preheating to 250°F prevents hydrogen in the weld.
• If you weld in a cold environment, preheat low-carbon castings to 75°F.
• If you are repairing a crack, drill both ends (1/8″) and remove the crack by grinding or gouging.

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The post How to Fix the Cracks Inside Cast Steel Gate Valve by Welding appeared first on ZECO Valve.

So have you heard of PWHT? What’s PWHT?

Overview of Post Weld Heat Treatment (PWHT)

Welding is an essential part of operating and maintaining assets in the petroleum (upstream, midstream, downstream) and chemical processing industries. While it has many useful applications, the welding process can inadvertently weaken equipment by imparting residual stresses into a material, leading to reduced material properties.

In order to ensure the material strength of a part is retained after welding, a process known as Post Weld Heat Treatment (PWHT) is regularly performed. PWHT can be used to reduce residual stresses, as a method of hardness control, or even to enhance material strength.

If PWHT is performed incorrectly, or neglected altogether, residual stresses can combine with load stresses to exceed a material’s design limitations. This can lead to weld failures, higher cracking potential, and increased susceptibility to brittle fracture.

PWHT encompasses many different types of potential treatments:

Two of the most common types are post heating and stress relieving:

Post Heating:

Hydrogen induced cracking (HIC) often occurs when high levels of ambient hydrogen permeate into a material during welding. By heating the material after welding, it is possible to diffuse hydrogen from the welded area, thus preventing HIC. This process is known as post heating and should begin immediately after the weld is completed. Rather than being allowed to cool, the material needs to be heated to a certain temperature depending on the type and thickness of the material. It should be held at this temperature for a number of hours dependent on the thickness of the material.

Stress Relieving:

By the time it’s complete, the welding process can leave a large number of residual stresses in a material, which can lead to an increased potential for stress corrosion and hydrogen induced cracking. PWHT can be used to release these residual stresses and reduce this potential. This process involves heating the material to a specific temperature and then gradually cooling it.

Whether or not material should undergo PWHT depends on a number of factors, including things like its alloying system or whether it’s been subject to heat treatment previously. Certain materials can actually be damaged by PWHT, while others almost always require it.

In general, the higher the carbon content of a material, the more likely it needs PWHT after welding activities have been conducted. Similarly, the higher the alloy content and cross-sectional thickness, the more likely the material is to need PWHT.

When PWHT is required for Carbon Steel?

Post Weld Heat Treatment or PWHT of Carbon Steel must be performed after every welding in order to ensure the material strength of the part is retained. The exact criteria for PWHT of carbon steel are mentioned in the ASME BPVC code. PWHT ensures the reduction of residual stresses, controlling material hardness, and enhancement of mechanical strength.

Advantages of Post Weld Heat Treatment | Purpose of PWHT

If PWHT is neglected or performed incorrectly, the residual stresses can combine with service load stresses. The value may exceed a material’s design limitations leading to weld failures, higher cracking potential, and increased susceptibility to brittle fracture. Other benefits of PWHT are listed below:

• Improved metallurgical structure
• Improved ductility of the material
• Reduced risk of brittle fracture as ductility increases
• Relaxed thermal stress due to the redistribution of residual stresses.
• Tempered metal
• Removal of diffusible hydrogen which helps in preventing Hydrogen-Induced Cracking (HIC)

PWHT Method and Equipment

The local post-weld heat treatment of the welded joints on the pipes shall be carried out by the electric-resistance method after the completion of all welding or repairing operations.

•  The resistance heater is electrically and thermally self-insulated and is built to size for each individual pipe.
• The applied voltages across the coils are either 220 or 380 volts AC depending on the power requirements.
•  The power controlling panel of post-weld heat treatment is composed of:
• A temperature controller indicator and recorder of digital type.
• A potentiometer device that controls the percentage of power input to the coils.
• A switch On and Off indicator lights, input, and output terminals for power and thermocouple connection.
• Electrical power contactors of the proper rating.
• Each panel will supply a single heating station and therefore for each heating operation, one panel will be needed. Heating and cooling rates are adjusted by manual selection of the percentage of power input by means of potentiometers.

Requirements for Post Weld Heat Treatment or PWHT

Before applying for the detailed PWHT requirements and exemption in these paragraphs, satisfactory weld procedure qualifications of the welding procedures specification to be used shall be performed in accordance with all the essential variables of ASME SECTION IX including conditions of post-weld heat treatment and other restrictions listed below.

While carrying out local post-weld heat treatment, the technique of application of heat must ensure uniform temperature attainment at all points of the portion being heat treated. Care shall be taken to ensure that the width of the heated band on either side of the weld edge shall not be less than four (4) times pipe thickness or 2″ whichever is greater.

Throughout the cycle of post-weld heat treatment, the portion outside the heated band shall be suitably wrapped under insulation so as to avoid any harmful temperature gradient at the exposed surface of the pipe. For this purpose, the temperature at the exposed surface of the pipe should not be allowed to exceed 400°c.

The valves, instruments, and other special items with welding ends, shall be protected,   because of the risk of damage during post-weld heat treatment.

No welding shall be performed after PWHT.

Automatic temperature recorders that have been suitably calibrated shall be employed. The calibration chart of each recorder shall be submitted to the owner prior to starting the heat treatment operations and his approval shall be obtained. Recording equipment shall be calibrated at least once every 12 months. Also, the instrument equipment (potentiometer) which is used for calibration of recorders should be supported by a related certificate.

Preparation and Attachment of Thermocouple for PWHT

After performing visual inspection and removing surface defects and temporary tack welds (if any) an adequate number of thermocouples (based on the diameter of pipes) shall be attached to the pipe directly and equally spaced location along the periphery of the pipe joint. The minimum number of thermocouples attached per joint shall be 1 for up to 3″ diameter, 2 for up to 6″ diameter, and 3 for up to 10″and 4 up to 12″diameter and above. However, the required minimum number of thermocouples to be attached can be increased if it is found necessary.

The thermocouples shall be placed on the joint and in firm contact with the pipe as near as possible to the weld area. Thermocouples should be directly tack welded to the joint or heating band jointly provided that they have a tail of the same material and an approved filler wire or electrode not larger than 2.5 mm in diameter is used for tack welding.

In order to avoid incorrect temperature readings due to direct radiation to thermocouples, it shall be protected by ceramic fiber blanked or any other suitable insulation material.

Heating resistance elements shall be laid over the attached thermocouples throughout of heating band and shall be insulated as shown in Fig. 1 below.

Insulating materials shall be mineral wool/glass wool which can overcome the temperature employed. The minimum insulation thickness shall be 50 mm. To hold the insulation material in position wire mesh shall be wrapped around and tied or tied by other suitable means.

PWHT Temperature, Time Record      

The post-weld heat treatment temperature and time and its heating and cooling rates shall be recorded automatically and present the actual temperature of the weld area. Each thermocouple shall be connected to the controlling and recording instrument for each treated joint.

Heating, Holding, and Cooling in PWHT

The heating temperature above 300°c shall be recorded and the heating and cooling rate shall not be more than that specified in related WPS and standards but in no case, more than 200°c/hr, and the difference between the temperatures measured by various thermocouples shall be within the range specified.

The heat treatment socking temperature and holding time shall be as specified in related welding procedure specifications. For easy reference, the values for different types of steel are given in the following table.

The cooling down to 300°c shall be controlled cooling. Below that the cooling down to ambient temperature shall take place under insulation coating without controlling.

Few Important Notes related to PWHT

PWHT operation shall be performed only by trained personnel having a similar experience and approved by the owner.

During PWHT joints shall be protected from rain and wind by adequate rain cover and windshield.

Hardness tests after PWHT shall be performed to determine if heat treatment has been performed effectively. Normally for Carbon Steel, the maximum Brinnel Hardness is 200 HB.

Safety Precautions during PWHT

The following safety precautions shall be provided during PWHT:

Equipment and panels shall be properly earthed.

Electrical technicians shall work with proper safety wears such as rubber gloves, shoes, etc.

Only a certified electrician will work.

Joints under PWHT shall be well cordoned with red tape/red light and danger display to avoid unknown persons coming in contact with high voltage electrical connections.

An adequate platform shall be made for in situ joints to avoid fall of a person.

Based on the above, PWHT is the critical process needed for welded material, which should be paid enough attention to during production. 

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High pressure Power Plant valve fish scale weld-TIG

For the fish scale weld showed in competition,also named TIG( Tungsten invert gas welding)， is a kind of welding technology.  It is named for its scale-like welding plane. Its characteristic is that the pipe is directly welded to the pipe. The main thing is to select the welding point, electrify, knock out the arc with the welding rod head, melt the flux in the welding rod, and then swing the welding clamp to make the welding center melt evenly in the welding position. Should be because the general welding effect is good, it will be like fish scale, so called fish scale welding in china.

ZECO welders exchanged welding skills with each other:

1. TIG welding itself belongs to open-arc operation. It can observe the shape and flow of molten pool very well, which is much superior to electrode arc welding.

2. Controlling the hand stability during welding is the key to avoid shivering and burning the dock pole causing tungsten inclusion in the molten pool. The control method can use the index finger of the welding torch to support the welded pipe or plate. Tungsten pole extension length can be selected according to the depth of groove, generally 3 ~ 5MM.

3. The wire transport method can be selected according to the size of the groove. When the groove angle is small, the welding wire can be placed in the middle of the solution pool and continuously fed into it. When the groove is large, two side points can be used to feed the wire (to be quite skilled, avoid touching the dock pole). The welding torch moves left and right to make the edge fuse well.

4. On the surface of the full scale, according to the scoring standard of the test competition, the residual height of TIG weld is generally 0-2MM, and the surface is smooth without undercut, blowhole, crack and fusion.

While doing welding process, we come across various types of welds. Sometimes we can’t understand a specific weld type or you know very less about that. But don’t worry by reading this article you will get an exact idea about each type. In this article, you will get to know each and every type. So, what are you waiting for? Let’s have a close look….

Following are the various types of welds:

1. Fillet Weld

A fillet weld is used to join two metal pieces together at right angles or at an angle. Fillet weld is commonly known as tee joint or lap joints weld.

In T- joint, the two metal pieces are joined at the right angle to each other and in lap joints, the two metal pieces overlap each other and welded at the edges.

The weld obtained in the fillet weld is triangular in shape that may contain convex, concave, or flat surfaces depending upon the welder’s technique.

A fillet weld is used to join flanges to pipes and cross-section of infrastructure. It is also used for the welding cross-section when the bolts are not enough strong to hold it and wear off easily.

2. Groove Weld:

Groove weld is defined as an opening between the two joint members which provides the space to contain the metal. Groove welds are the most used welds after the fillet weld. There are seven basic types of groove weld:

• Square groove weld
• Single-V groove weld
• Single-bevel groove weld
• Single-U groove weld
• Single-J groove weld
• Flare-V weld
• Flare-bevel weld

3. Surfacing Weld:

The process of deposition of a metal on another metal in order to get desired properties and dimensions with the help of a welding technique is called surfacing welding. And the weld we obtained is called surfacing weld.

4. Plug Weld

A plug weld is used to lock two metal pieces together. It is mostly used if we want to join metal with different thicknesses. These types of weld are used when we have to weld inside a pipe. The shape of the plug weld is circular.

5. Slot Weld

Basically, a slot weld join one surface of a metal to the other through an elongated cavity or hole. This cavity or hole may be partially filled with the weld metal or may be open at one end.

6. Flash Weld

Flash welding gives us a flash weld. While making the flash weld, there is no use of any type of filler metals. Here, the fusion is generated over the complete abutting (adjoining) surface. Here, heat is generated with the help of resistance to the current flow between the surfaces, and also pressure is applied to generate heat. This type of weld is used in the railroad industry.                                                                  

7. Seam Weld

Seam weld is a continuous weld that is obtained between two overlapping members of a joint. Seam welding is obtained by using resistance seam welding.

8. Spot Weld

The weld that we get by doing spot welding is called spot weld. In this welding technique, the two metal pieces are joined together as spots. The adjoining surfaces of two metals have spotted at a particular distance. Look at the diagram above to understand it clearly.

In this article, we learned about the different types of welds. Hope this article gives you clear information about the different weld types. If you got some worthy knowledge out of it, share it with your friends because “Sharing is Caring”

‘Fish-scale’ is a technique of welding, known as its cataphracted weld joint. It’s regarded as the best welding technique of the world.

It’s a TIG welding method that through swing of wrist make welding nozzle rolling as the scalloped shape during welding process, it’s also known as ‘swing welding’.

How to weld a good fish scale?

1. The center of gravity  

Welding is like martial arts, the first thing to do is the chassis to be stable, that is, “horse step” to be stable, not the center of gravity is not stable when shaking welding is difficult to weld a good weld.

2. The welding torch  

Hand shake things can cause the formation of tungsten electrode burning molten pool the phenomenon of tungsten, weld edge is not neat, the scale size not neat, we can through the gun hand little finger and ring finger for stabilizing gun control, contact with the product can also be spread by argon arc welding of ceramic nozzle, abutting on the parts and then adjust the length of the tungsten electrode according to the depth of the crater is probably between 3 to 5 mm.

3. The wire feed stability  

The method of wire feeding is adjusted according to the size of the weld groove. If the groove is small, the welding wire can be continuously fed in the center of the weld pool. When the width of the weld is large, the way of wire feeding is adopted.

“Accurate” “three accurate”

1. The parameters  

Welding parameters are the key to the quality of welding, and it is very necessary to choose the correct welding parameters.  Flat welding, vertical welding, according to the actual station, the actual plate thickness to choose the appropriate parameters and welding material specifications, welding current is small is not easy to arc, welding current is large is easy to weld through, molten iron next trip.

2. Accurate Angle and position  

Welding gun Angle, welding position will affect the final welding molding, at the same time to avoid the emergence of welding defects (tungsten, no fusion, slag).  General flat butt welding electrode swing method has zigzag, crescent, triangle, ring, and figure eight!  The key of vertical fillet welding is how to control the molten pool metal. The electrode should swing up and down rhythmically according to the cooling condition of the molten pool metal.

3. time

In the welding process, when the first molten pool appears after arc initiation, the arc should be raised quickly.  When the molten pool is instantly cooled into a dark red point, the arc is lowered to the arc pit, and the droplet falling overlaps with the molten pool in front by 2/3, and then the arc is raised.  This rhythmically forms a fillet weld.

Method of handling rod

TIG welding is an open arc operation, which can observe the shape andflow of weld pool very clearly. The key is to keep your hand very stable and steady in welding!     

Control method: Use the index finger that holding torch to support the welding plate. The length of tungsten electrode could be choose as per the depth of crevasse, usually is 3 – 5mm.

Method of handling wire

The method of handling wire should be choose as per the size of crevasse. When the size is small, welding wire could place at the middle of welding pool and continously ‘feed-in’; When the size is big, could feed the wire at two sides bit by bit, and the torch should move aroud to make sure the edge fuse well.

Welding tips

To evaluate an beautiful fish-scale welding effect, the excess weld metal of TIG welding bead should be 0 – 2mm, and the surface should be smooth. The angle, and welding current are quite important! The width and thickness should keep the same when rod moved backward, usually we make it moved upward as ‘V’ shape in vertical welding.

Plus the current is last but not least, the rod is easy to stick with plate when the current is too small; yet the plate will be punchthrough when surrent is too big. Hence current is always a hot potato to green hands or freshmen. How to deal with it when we are unskilled? JSKWED make you weld easy and easy, to press 3 seconds into our ‘simple mode’.

Which Gas Do You Need for Your TIG Weld?

No matter what you’re planning to do, your TIG weld always needs to be protected by a shielding gas. No gas and you’re going to have a very bad time. But which one works the best?

TIG gases

TIG welding will only work with inert (noble) gases. There are six inert gases, but only two of them are cheap enough to be used: argon and helium. That means picking a shielding gas is relatively simple. Pure argon is used for every application: mild steel, stainless steel and aluminium.

Helium can also be used, but it’s more expensive, so it is usually only added to argon as an extra to give more penetration. For example, if you were welding on really thick metal and you were reaching the limits of your welder’s amperage range, you could swap to an Ar/He mix. Helium is common in colder climates as well, as it burns hotter, so it provides more heat and penetration for the weld.

Why can’t semi-inert (active) gases be used?

Inert gases have only one function when being used to shield a weld: shielding the weld. They’re denser than the surrounding air, so they can completely block out any oxygen or otherwise, that would contaminate the weld. They have no effect on the welding process, and there are no changes to the arc.

On the other hand, active gases like CO2 and oxygen do have an effect on the weld. They can alter the characteristics of the arc, as well as aid with penetration, so they do more than just shield the weld.

Unlike MIG welding, which benefits from having active gases mixed in, TIG is not so forgiving. Because CO2 and oxygen affect the weld directly, it reacts badly, and the added heat can blow holes through the workpiece, plus it burns up the tungsten. By only using an inert gas like argon, the weld and your tungsten will both be properly protected.

Shielding gas protection

Shielding gases for MIG/GMAW welding

The basic gas for MIG/MAG welding is argon (Ar). Helium (He) can be added to increase penetration and fluidity of the weld pool. Argon or argon/helium mixtures can be used for welding all grades. However, small additions of oxygen (O2) or carbon dioxide (CO2) are usually needed to stabilize the arc, improve the fluidity and improve the quality of the weld deposit. For stainless steels there are also gases available containing small amounts of hydrogen (H2).

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