When choosing a casting material for a project, it’s important to understand the key differences between cast iron (which is also called gray iron) and ductile iron castings. Both materials have their own advantages and disadvantages that should be taken into account.

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In general, cast iron is best suited for applications where strength and hardness are more important than ductile properties, while ductile iron is best suited for applications where ductility is more important than strength and hardness.

Cast Iron vs. Ductile Iron: The Main Differences 

Cast iron are made with carbon and silicon as the main alloying elements. The carbon creates flakes within the structure of the metal which gives it its unique gray color.

Cast iron is known for its good damping capacity, meaning it can absorb shock without deforming. It also has good resistance to abrasion, making it ideal for applications where there is a lot of friction.

For these reasons, cast iron is often used in the automotive industry for engine blocks and cylinder heads. Another advantage of cast iron is that it is easier to machine than other types of iron, making it a cost-effective choice for many applications.

Ductile iron is made with carbon, silicon, manganese, and molybdenum as the main alloying elements. The addition of manganese and molybdenum makes ductile iron more resistant to wear and tear than cast iron. It also has higher tensile strength and ductility, which means it can withstand greater stress without breaking.

For these reasons, ductile iron is often used in construction applications such as pipes and valves. It is also used in the Automotive industry for suspension components such as tie rods and control arms.

Ductile iron can be more expensive than cast iron due to the addition of manganese and molybdenum, but its increased strength and durability make it worth the investment for many applications.

Cast Iron vs. Ductile Iron: Different Grades

Grades of Cast (Gray) Iron

The higher the grade number, the higher the carbon content.

Grades of Ductile (Nodular) Iron

These numbers represent percentages by weight of carbon (C), silicon (Si), and manganese (Mn). Just like with cast iron, the higher grades have more carbon and are therefore stronger and harder but more brittle as well.

It should be noted that there are many other alloys that can be added to both cast iron and ductile iron to alter their properties. These alloys include chromium, molybdenum, nickel, vanadium, etc. Adding these alloys can increase strength, hardness, abrasion resistance, corrosion resistance. But will also increase cost.

Cast Iron vs. Ductile Iron: Application Areas

Cast iron is commonly used in applications where high strength is not required but wear resistance and low cost are important considerations such as: machine tool beds, gears, rollers, crankshafts, pump housings, valves, valve seats and wedges, pressure vessels, manifolds, pistons, piston rings and cylinders, brake drums and discs, differential cases and covers.

Ductile iron is commonly used in many of the same applications as cast iron but is typically used when higher strength or improved ductility is required such as: automotive suspension components, hydraulic cylinder barrels, crankshafts, connecting rods, gears transmission shafts and axles.

Cast Iron vs. Ductile Iron: Production Process

Cast iron are made by pouring molten pig iron into sand molds. Once the pig iron has cooled and hardened, it’s ready to be used in whatever application it was intended for. The main benefit of cast iron is that it’s very strong and durable.

Cast iron also has good vibration dampening qualities, which makes it ideal for use in applications where there will be a lot of movement or vibrations.

Ductile iron castings are made using the same basic method as cast iron. The difference is in the composition of the pig iron.

In order to create ductile iron, manufacturers add spheroidizer containing magnesium or cerium to the pig iron before pouring it into the mold.

This addition gives ductile iron its increased ductility over cast iron. The increased ductility of ductile iron makes it an ideal choice for applications that require flexibility or elongation.

Cast Iron vs. Ductile Iron: Machinability

One of the biggest difference between cast iron and ductile iron is machinability—or how easy or difficult it is to machine a particular type of iron.

In general, cast iron is easier to machine than ductile iron.

The reason for this is that cast iron has a lower carbon content than ductile iron, which means that there are fewer carbon particles in the metal that can cause obstacles during the machining process.

However, even though cast iron is generally easier to machine than ductile iron, there are still some challenges that need to be taken into account. For example, cast iron tends to be more brittle than ductile iron, which means that it can break more easily during the machining process.

In addition, cast iron also has a higher graphite content than ductile iron, which can cause issues with clogging and build-up during the machining process.

Cast Iron vs. Ductile Iron: Surface Finish Quality

Another important consideration when choosing between cast iron and ductile iron is surface finish quality—or how smooth and polished the final product will be. When it comes to surface finish quality, ductile iron definitely has the upper hand over cast iron.

The reason for this is that, because of its higher carbon content, ductile iron can be heat-treated in such a way that its surface becomes harder and smoother. This results in a much higher-quality surface finish when compared to cast iron.

Cast Iron vs. Ductile Iron: Heat Treatments

Cast iron castings are best suited for applications where high temperatures are not a concern. In general, cast iron can withstand temperatures up to °F (650°C). When cast iron is heated above this range, the graphite flake structure begins to deteriorate, which can lead to premature failure. For this reason, gray iron is not typically used in high-temperature applications.

Ductile iron, on the other hand, is best suited for applications that require high temperatures. The microstructure of ductile iron is more resistant to degradation at high temperatures than gray iron. As a result, ductile iron can withstand temperatures up to °F (980°C).

When it comes to heat treatment, both cast iron and ductile iron can be annealed, normalized, quenched, and tempered. However, because of its higher carbon content, cast iron is more difficult to weld than ductile iron. Ductile iron can also be alloyed with chromium, molybdenum, and nickel to improve its weldability.

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Cast Iron vs. Ductile Iron: The Cost

Cast iron castings are less expensive than ductile iron castings. That’s because they’re easier to produce and don’t require as much material. Cast iron is also more resistant to wear and tear than ductile iron, which means it will last longer before it needs to be replaced or repaired. However, cast iron is more brittle than ductile iron, which means it’s more likely to break or crack under stress.

Ductile iron is more expensive than gray iron because it’s more difficult to produce and requires more material. However, its increased strength and durability means it will last longer before it needs to be replaced or repaired. Ductile iron is also less likely to break or crack under stress than cast iron, making it the ideal choice for applications where safety is a concern.

For cast iron, the average price per pound is USD$0.60 (USD$1.32/kg). For ductile iron, the average price per pound is USD$0.85 (USD$1.87/kg). This means that, on average, ductile iron costs 40% more than cast iron.

It should be noted that I have visited no less than 50 foundries in China. The quality of the castings produced also varies. Some manufacturers produce ductile iron for less than $1/kg, which is less than $0.45 per pound. Such prices look very attractive, but for the quality of the product to put a question mark.

If you’re looking for the most economical option, then cast iron castings are the way to go. But if you need a casting that’s stronger and more durable, then you’ll need to pay more for ductile iron.

In either case, it’s important to consider the total cost of ownership when making your decision so that you can choose the option that’s best for your needs and budget.

Key Considerations for Sourcing in China

As a buyer, you always want to get the best value for your money. When it comes to purchasing cast iron and ductile iron castings from China, there are a few things you should keep in mind in order to ensure you are getting a high quality product.

1.Certification

Check if the company has ISO certification. This certification means that the company has meets the standards set by the International Organization for Standardization and is committed to providing high quality products and services.

More important than ISO certification, is discharge permit. A discharge permit is a basic requirement for operating a foundry in China. If your potential foundry supplier is not equipped with this document, there is a risk of stopping production in the middle of the process, so you need to pay more attention.

2. Pricing

When it comes to pricing, there is a wide range of prices quoted by suppliers in China. This is because there are a number of factors that can affect the price of iron castings, such as the size and complexity of the casting, the quantities required, and the lead time. As a result, it’s important to get quotes from several different suppliers so that you can compare prices and find the best deal.

3. Quality Control

Quality control is another important consideration when purchasing iron castings from China. There are a number of ways to ensure that you’re getting high-quality products, including working with a reputable supplier, conducting audits at the factory, and requesting samples before placing an order.

Purchasing ductile iron castings from China can be a great way to save money on your procurement costs. However, there are a few things you need to keep in mind in order to ensure that you’re getting high-quality products at a fair price.

Be sure to look for certification, get quotes from multiple suppliers, and conduct quality control checks before making your purchase. With these considerations in mind, you can be confident that you’re getting the best value for your money when sourcing ductile iron castings from China.

Water Gate Valve - A Guide

Figure 1: Gate valve

Water gate valves are suitable for regulating water flow in commercial and residential water systems. Due to the diversity of gate valve designs and applicable materials, gate valves can operate well in a wide range of water applications, including plumbing systems. This article will take a closer look at gate valves and offer advice on how to use them effectively in water systems. For a more comprehensive gate valve understanding, read our article on gate valves!

Table of contents

• Water gate valves operating principle
• Types of water gate valves
• Water gate valve materials
• Advantages and disadvantages
• Gate valve water applications
• Other valves for water applications
• FAQs

Water gate valves operating principle

Water gate valves are multi-turn valves that control the flow of water. The valve’s handwheel (Figure 2 labeled A) needs to turn more than 360° for the valve to fully open or close. When opening, stop turning the handwheel when there is resistance. If there is too much resistance, loosen the packing nut. Be sure to tighten it again before operating the valve.

The handle turns clockwise to close the valve. Applying a lot of pressure in an attempt to tighten the seal may cause permanent damage to the valve. If debris accumulates in the valve body, it may not close completely. In this case, open a faucet within the building and work the handle (open and close) to flush out debris and close the water main gate valve. Read our article on troubleshooting and repairing gate valves to learn more.

The stem (Figure 2 labeled B) can be rising or non-rising, meaning it rises out of the valve body when the valve opens or it doesn’t. Also, gate valves are capable of pneumatic, hydraulic, and electric actuation.

When the gate valve is open, water can flow through the valve unimpeded with almost no pressure drop. As the valve closes, its gate (Figure 2 labeled G) slides into the water flow until it completely blocks it. A gate valve is unsuitable for modulating or throttling flow; a partially open or closed valve causes turbidity, which significantly wears down the valve.

The valve body (Figure 2 labeled E) and bonnet (Figure 2 labeled D) protect the inner mechanisms from outside elements and help prevent leaking. The gasket (Figure 2 labeled C) protects the packing and helps stop leaking along the stem. A gate valve’s flanges (Figure 2 labeled F) connect to the piping. Overall, this standard design helps gate valves seal tight and stay leak-free when used in water applications.

Figure 2: Gate valve components: handwheel (A), stem (B), gasket (C), bonnet (D), valve body (E), flange (F), and gate (G).

Water gate valve materials

The following is not a complete list of gate valve body and seal materials. When considering between materials, consult our chemical resistance guide to learn more. Common water gate valve materials for different applications are:

• Residential and commercial applications: Residential and commercial water systems typically use PVC, CPVC, ABS, brass, or bronze valves because these are cheaper options. The plastic valves, specifically, do not corrode. These materials hold up under most if not all temperature and pressure conditions in a residential water system.
  • Valve body
    • PVC: Working temperature 0 - 60 °C (140 °F); Schedule 80 PVC handles higher pressures and is suitable for residential applications
    • CPVC: Working temperature up to 92 °C (198 °F); lead-free so suitable for potable water systems
    • ABS: Working temperature -20-80 °C (-4 to 176 °F); working pressure up to 15 bar (217 psi).
    • Brass and Bronze: The working temperatures and pressures of brass and bronze valves are controlled by the seal material. Brass is more cost-effective upfront, but is also more susceptible to corrosion, specifically dezincification, if there is more than 15% zinc in the brass.
    • Stainless steel: Stainless steel stands out among the above materials because it is not cost-effective upfront. However, stainless steel is highly resistant to corrosion and can operate in a wide range of temperatures and pressures. Also, stainless steel has a life expectancy of up to 50 years.
  • Valve seal
    • Metal-to-metal: Metal-to-metal seals withstand higher temperatures but also have higher leakage rates and are less suitable for residential and commercial applications.
    • Non-metal: EPDM, NBR, and FKM seals are all suitable for residential and commercial applications. The lowest maximum working temperature among these materials (shared by EPDM and NBR) is 120 °C (248 °F).
• Industrial applications: Industrial valves typically require higher temperature, pressure, and chemical resistance than valves used in residential and commercial water systems.
  • Valve body
    • Cast iron: Some cast iron has a working temperature up to °C ( °F); working pressure depends on the pressure class, the two most common being class 125 and class 250, rated for 13 bar psi) and 34 bar (493 psi) respectively at less extreme temperatures.
    • Ductile iron: Working temperature up to 730 °C; pressure classes 150 and 300, rated for 17 bar (246 psi) and 44 bar (638 psi) respectively.
    • Forged steel: Working temperature from -29 °C (-20.2 °F) to 425 °C (797 °F); a class 800 forged steel valve has working pressure up to 138 bar ( psi).
  • Valve seal
    • PTFE: PTFE (Teflon) seals have a working temperature up to 260 °C (500 °F). Applications with temperatures above this require metal-to-metal seals.
• Rust and corrosion resistance
  • Rust and corrosion can cause health issues and damage to plumbing.
  • Plastic valves do not corrode or rust.
  • Stainless steel and bronze have corrosion resistance, but certain acids and chemicals can corrode these materials.
    • Bronze: formic acid vapor, sodium chloride, or sulfur dioxide
    • Stainless steel: Stainless steel valves can corrode when exposed to the following for a long period of time: damaging chemicals, heat, moisture, saline, or grease.

Advantages and disadvantages

Advantages

• Water gate valves are versatile and have several uses.
• Durability and reliability
• Ease of operation
• Available in a variety of sizes and designs
• Insignificant pressure drop across the valve
• Bidirectional
• Suitable for high-pressure and temperature applications

Disadvantages

• Prone to leaking if not maintained properly
• Cannot be used for throttling
• Very noisy when partially open or closed

Gate valve water applications

• Water main gate valve: Gate valves are the most common type of valve used for opening and closing water flow in a water main. Primarily, water main gate valves are used to isolate parts of the system for maintenance.
• Irrigation systems: Gate valves permit large volumes of water and are ideal for irrigation systems. They are the primary shut-off valve between the irrigation system and the water supply. They can also isolate flows within the system.
• Water distribution systems: A gate valve used in a water distribution system is also referred to as a water service valve. Because these valves are typically in underground piping systems, a non-rising stem gate valve is the typical type used. The valve’s primary function in this system is to isolate flows.
• Industrial water systems: Industrial water is water used in a wide range of industries for purposes such as washing, diluting, and transporting. This water typically contains more suspended solids, making gate valves ideal due to their full-bore design.
• Residential and commercial plumbing systems: Gate valves close the main water supply in residential and commercial plumbing. They are also shut-off valves on main and branch water supplies.
• Wastewater treatment plants: Gate valves are shut-off valves in wastewater treatment applications. Knife gate valves are especially applicable due to their disc’s knife-like edge, which can cut through slurries and other solids in media.

Other valves for water applications

Beyond gate valves, there are several types of valves used for water applications. Each type has specific functionality to consider when designing a water system.

• Check valve: There are several types of check valves, including ball, butterfly, and disc check valves. Check valves allow fluid flow in only one direction, preventing backflow.
• Butterfly valve: Butterfly valves are cost-effective at large sizes and take up less space than other valve types. These valves are commonly used in irrigation systems.
• Globe valve: Globe valves are similar to gate valves in that they are multi-turn valves. However, globe valves permit flow modulation and therefore are suitable for precise flow control needs. Read our globe valve vs gate valve article to learn more about their similarities and differences.
• Pressure relief valve: Pressure relief valves open when pressure in a water system rises above a set limit. Boiler systems use pressure relief valves to prevent damage to the overall system.
• Ball valve: Ball valves have replaced gate valves in many applications. Ball valves typically have better sealing properties and are easier to close. The primary disadvantage of ball valves is that they are more expensive than gate valves.

FAQs

What is a water gate valve?

A gate valve used in water applications has excellent sealing properties and is relatively inexpensive compared to many valve types.

How do you open a gate valve?

The proper way to open a gate valve is to turn the handle to the left until you feel resistance. It takes about six turns to open a water gate valve fully.

Can water gate valves be used for throttling?

Gate valves cannot throttle water. There is no close relationship between the size of the opening left by a partially closed disc and the amount of water that flows through the valve.

How do you tell whether a water gate valve is open or closed?

Rising stem gate valves indicate whether they are open or closed based on the height of the handle. Non-rising stem gate valves have a position indicator that shows the position of the gate.

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